The program mutation is a testing technique that assesses the quality of test input data by examining whether the test data can distinguish a set of alternate programs (representing specific types of faults) from the program under test. We have extended the conventional mutation method to be applicable for object-oriented (OO) programs. The method, termed Class Mutation, is a form of OO-directed selective mutation testing that focuses on plausible flaws related to the unique features in OO (Java) programming. This paper introduces the Class Mutation technique and describes the results of the case study performed to investigate the applicability of the technique.