Aspect-Oriented Programming (AOP) has introduced a complementary set of mechanisms which enhance the modularisation of crosscutting concerns. However, such mechanisms represent new potential sources of faults that may be systematically tackled with mutation testing. In this demonstration we present a tool, named Proteum/AJ, which automates the mutation testing of AspectJ programs. Proteum/AJ supports the main steps of this testing approach and realises a set of requirements for mutation-based testing tools like mutant handling, test case handling and mutant analysis. Our experience in using the tool provided us with evidence on the feasibility of performing mutation testing of AO programs. In this demonstration we are going to share some of this experience with the audience.