The potential carcinogenicity of benzidine (92875) based dyes is discussed. Data from animal experiments, case reports, and other sources on the metabolism and carcinogenicity of benzidine, o- tolidine (119937) and o-dianisidine (119904) based dyes is presented. Recommendations for employees, employers, and physicians are made for the control of exposure to these dyes in the workplace. Appendices list benzidine, o-tolidine, o-dianisidine dyes, the amounts produced in the United States, amounts imported, uses of the dyes, and the estimated number of exposed workers. The authors conclude that benzidine based dyes are potential human carcinogens.