The toxic hazards associated with exposure to 3,3'-dichlorobenzidine (91941) (DCB) are discussed. The use of DCB as an intermediate in the production of organic pigments is described. Problems in evaluating DCB carcinogenicity directly among occupationally exposed populations are reviewed, including concomitant exposures to other chemicals and very brief periods of exposure. Research finding are summarized from numerous studies of DCB carcinogenicity in laboratory animals such as rats, mice, and hamsters. DCB related tumors of the external ear duct, mammary tumors, injection site tumors, hepatic tumors, hematopoietic tumors, sebaceous papillomas, squamous cell carcinomas, fibroadenomas, adenocarcinomas, salivary gland tumors, spindle cell sarcomas, polymorphic cell sarcomas, and pulmonary adenomas are described. DCB metabolism is discussed, and differences between the metabolic pathway of DCB compared to that of benzidine (92875) and beta-naphthylamine (91598) are noted. The recovery of DCB in the urine of exposed Japanese workers is examined, and the working conditions in Russian industries using DCB are discussed. The author concludes that DCB has been proven to be carcinogenic in hamsters, rats, and mice; however, conclusive evidence of DCB carcinogenicity in humans cannot be obtained because of the lack of populations exposed only to DCB.