diff --git a/graphene/core/fields.py b/graphene/core/fields.py
index 684a61cf..275b08f3 100644
--- a/graphene/core/fields.py
+++ b/graphene/core/fields.py
@@ -134,7 +134,7 @@ class Field(object):
         object_type = self.get_object_type(schema)
         if object_type and object_type._meta.is_mutation:
             assert not self.args, 'Arguments provided for mutations are defined in Input class in Mutation'
-            args = object_type.input_type.fields_as_arguments(schema)
+            args = object_type.get_input_type().fields_as_arguments(schema)
 
         internal_type = self.internal_type(schema)
         if not internal_type:
@@ -151,11 +151,13 @@ class Field(object):
         else:
             resolver = self.resolve
 
-        field_type = GraphQLField
-        if object_type and issubclass(object_type, InputObjectType):
-            field_type = GraphQLInputObjectField
+        if issubclass(self.object_type, InputObjectType):
+            return GraphQLInputObjectField(
+                internal_type,
+                description=description,
+            )
 
-        return field_type(
+        return GraphQLField(
             internal_type,
             description=description,
             args=args,
@@ -164,7 +166,7 @@ class Field(object):
 
     def __str__(self):
         """ Return "object_type.name". """
-        return '%s.%s' % (self.object_type, self.field_name)
+        return '%s.%s' % (self.object_type.__name__, self.field_name)
 
     def __repr__(self):
         """
diff --git a/graphene/core/types.py b/graphene/core/types.py
index 93b0d0fa..d95ab4e8 100644
--- a/graphene/core/types.py
+++ b/graphene/core/types.py
@@ -23,7 +23,7 @@ class ObjectTypeMeta(type):
         return Interface in parents
 
     def is_mutation(cls, parents):
-        return Mutation in parents
+        return issubclass(cls, Mutation)
 
     def __new__(cls, name, bases, attrs):
         super_new = super(ObjectTypeMeta, cls).__new__
@@ -218,7 +218,9 @@ class ObjectType(six.with_metaclass(ObjectTypeMeta, BaseObjectType)):
 
 
 class Mutation(six.with_metaclass(ObjectTypeMeta, BaseObjectType)):
-    pass
+    @classmethod
+    def get_input_type(cls):
+        return getattr(cls, 'input_type', None)
 
 
 class InputObjectType(ObjectType):
diff --git a/tests/core/test_fields.py b/tests/core/test_fields.py
index c03a2532..028fba99 100644
--- a/tests/core/test_fields.py
+++ b/tests/core/test_fields.py
@@ -31,10 +31,7 @@ class ObjectType(object):
     def __str__(self):
         return "ObjectType"
 
-ot = ObjectType()
-
-ObjectType._meta.contribute_to_class(ObjectType, '_meta')
-
+ot = ObjectType
 
 schema = Schema()