The way the activity diagram in the Unified Modeling Language is currently defined is not object-oriented. Activity diagrams specify data/object flow. We feel that the object-oriented principles should hold for every part of the standard for object-oriented analysis and design. Therefore this paper proposes a few small changes to the UML metamodel that will integrate activity diagrams into an object-oriented scheme, focusing on three fundamental object-oriented principles: responsibility, information hiding/encapsulation, and delegation.