diff --git a/src/Executor/Executor.php b/src/Executor/Executor.php
index f767b98..81a895c 100644
--- a/src/Executor/Executor.php
+++ b/src/Executor/Executor.php
@@ -722,11 +722,11 @@ class Executor
      */
     private static function completeAbstractValue(ExecutionContext $exeContext, AbstractType $returnType, $fieldASTs, ResolveInfo $info, $path, &$result)
     {
-        $resolveType = $returnType->getResolveTypeFn();
+        $runtimeType = $returnType->resolveType($result, $exeContext->contextValue, $info);
 
-        $runtimeType = $resolveType ?
-            call_user_func($resolveType, $result, $exeContext->contextValue, $info) :
-            Type::getTypeOf($result, $exeContext->contextValue, $info, $returnType);
+        if (null === $runtimeType) {
+            $runtimeType = self::inferTypeOf($result, $exeContext->contextValue, $info, $returnType);
+        }
 
         // If resolveType returns a string, we assume it's a ObjectType name.
         if (is_string($runtimeType)) {
@@ -849,4 +849,25 @@ class Executor
 
         return self::executeFields($exeContext, $returnType, $result, $path, $subFieldASTs);
     }
+
+    /**
+     * Infer type of the value using isTypeOf of corresponding AbstractType
+     *
+     * @param $value
+     * @param $context
+     * @param ResolveInfo $info
+     * @param AbstractType $abstractType
+     * @return ObjectType|null
+     */
+    private static function inferTypeOf($value, $context, ResolveInfo $info, AbstractType $abstractType)
+    {
+        $possibleTypes = $info->schema->getPossibleTypes($abstractType);
+
+        foreach ($possibleTypes as $type) {
+            if ($type->isTypeOf($value, $context, $info)) {
+                return $type;
+            }
+        }
+        return null;
+    }
 }
diff --git a/src/Type/Definition/AbstractType.php b/src/Type/Definition/AbstractType.php
index 97fa1c3..dd27677 100644
--- a/src/Type/Definition/AbstractType.php
+++ b/src/Type/Definition/AbstractType.php
@@ -9,7 +9,12 @@ GraphQLUnionType;
 interface AbstractType
 {
     /**
-     * @return callable|null
+     * Resolves concrete ObjectType for given object value
+     *
+     * @param $objectValue
+     * @param $context
+     * @param ResolveInfo $info
+     * @return mixed
      */
-    public function getResolveTypeFn();
+    public function resolveType($objectValue, $context, ResolveInfo $info);
 }
diff --git a/src/Type/Definition/InterfaceType.php b/src/Type/Definition/InterfaceType.php
index 410de70..dfe65f6 100644
--- a/src/Type/Definition/InterfaceType.php
+++ b/src/Type/Definition/InterfaceType.php
@@ -19,11 +19,6 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
      */
     public $description;
 
-    /**
-     * @var callback
-     */
-    private $resolveTypeFn;
-
     /**
      * @var array
      */
@@ -51,7 +46,6 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
 
         $this->name = $config['name'];
         $this->description = isset($config['description']) ? $config['description'] : null;
-        $this->resolveTypeFn = isset($config['resolveType']) ? $config['resolveType'] : null;
         $this->config = $config;
     }
 
@@ -84,10 +78,19 @@ class InterfaceType extends Type implements AbstractType, OutputType, CompositeT
     }
 
     /**
+     * Resolves concrete ObjectType for given object value
+     *
+     * @param $objectValue
+     * @param $context
+     * @param ResolveInfo $info
      * @return callable|null
      */
-    public function getResolveTypeFn()
+    public function resolveType($objectValue, $context, ResolveInfo $info)
     {
-        return $this->resolveTypeFn;
+        if (isset($this->config['resolveType'])) {
+            $fn = $this->config['resolveType'];
+            return $fn($objectValue, $context, $info);
+        }
+        return null;
     }
 }
diff --git a/src/Type/Definition/Type.php b/src/Type/Definition/Type.php
index 52a1f63..02908b0 100644
--- a/src/Type/Definition/Type.php
+++ b/src/Type/Definition/Type.php
@@ -211,26 +211,6 @@ abstract class Type
         return $type;
     }
 
-    /**
-     * @param $value
-     * @param mixed $context
-     * @param AbstractType $abstractType
-     * @return Type
-     * @throws \Exception
-     */
-    public static function getTypeOf($value, $context, ResolveInfo $info, AbstractType $abstractType)
-    {
-        $possibleTypes = $info->schema->getPossibleTypes($abstractType);
-
-        foreach ($possibleTypes as $type) {
-            /** @var ObjectType $type */
-            if ($type->isTypeOf($value, $context, $info)) {
-                return $type;
-            }
-        }
-        return null;
-    }
-
     /**
      * @var string
      */
diff --git a/src/Type/Definition/UnionType.php b/src/Type/Definition/UnionType.php
index e113986..a5b9c9f 100644
--- a/src/Type/Definition/UnionType.php
+++ b/src/Type/Definition/UnionType.php
@@ -20,11 +20,6 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType
      */
     private $possibleTypeNames;
 
-    /**
-     * @var callback
-     */
-    private $resolveTypeFn;
-
     /**
      * @var array
      */
@@ -54,7 +49,6 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType
          */
         $this->name = $config['name'];
         $this->description = isset($config['description']) ? $config['description'] : null;
-        $this->resolveTypeFn = isset($config['resolveType']) ? $config['resolveType'] : null;
         $this->config = $config;
     }
 
@@ -85,10 +79,10 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType
                 $this->name
             );
 
-            // TODO: Return some sort of generator to avoid multiple loops
-            $this->types = Utils::map($types, function($type) {
-                return $type instanceof DefinitionContainer ? $type->getDefinition() : $type;
-            });
+            $this->types = [];
+            foreach ($types as $type) {
+                $this->types[] = Type::resolve($type);
+            }
         }
         return $this->types;
     }
@@ -113,10 +107,19 @@ class UnionType extends Type implements AbstractType, OutputType, CompositeType
     }
 
     /**
+     * Resolves concrete ObjectType for given object value
+     *
+     * @param $objectValue
+     * @param $context
+     * @param ResolveInfo $info
      * @return callable|null
      */
-    public function getResolveTypeFn()
+    public function resolveType($objectValue, $context, ResolveInfo $info)
     {
-        return $this->resolveTypeFn;
+        if (isset($this->config['resolveType'])) {
+            $fn = $this->config['resolveType'];
+            return $fn($objectValue, $context, $info);
+        }
+        return null;
     }
 }