This study was undertaken to shed light on problems concerning the construction and use of a common test battery for various South African population groups. Specifically, the psychometric suitability for use with Indian and Black pupils, of the Junior Aptitude Tests (JAT), which were standardized for White pupils in Standard 7 was studied. The predictive validity and construct validity of the JAT and the test items were also investigated. The sample consisted of: (1) 1,056 White students from 30 schools; (2) 1,063 Indian students from 30 schools; and (3) 1,093 Black students from 28 schools. The students in the sample ranged in age from 13 to 20 years. Bias in the JAT was studied through comparison with results on the Raven Standard Progressive Matrices of J. C. Raven et al. (1977) and results on a reasoning test and biographical questionnaire developed for the study. Most tests of the JAT battery were found to be suitable for Indian students, but not for Black students. Differences between the JAT means for White and Black students were generally two standard deviations and more, and test reliabilities were low. Although construct bias was not apparent, bias was found in many test items. This study illustrates the difficulties involved in assessment of the diverse population groups in South Africa. A 109-item list of references is attached. Seven tables in Appendix A, 7 in Appendix B, and 60 graphs in Appendix C provide data to support the report. (SLD)