The object-oriented paradigm has seen widespread acceptance by the software development community but the testing of programs written in object oriented languages is less well developed than the testing of programs written in conventional ones. This paper introduces Class Mutation which is a form of OO-directed selective mutation testing that provides a means of assessing how good developed test sets are for OO programs. Experimental results are given for the application of three particular mutation operators for the Java language to assess test sets developed to satisfy a conventional criterion.