Studies of the metabolic conversion of phenyl-beta-naphthylamine (135886) (PBNA) and 2-nitronaphthalene (581895) (2NNA) to the carcinogen, beta-naphthylamine (91598) (BNA) were reviewed. PBNA replaced BNA as an antioxidant in rubber when BNA was found to be associated with bladder cancer and is widely used in the rubber industry with an estimated 15,000 workers exposed. Commercial grade PBNA contains 20 to 30 parts per billion BNA as a contaminant. Studies have shown that PBNA undergoes metabolic conversion to BNA in human and dogs. The acute and chronic effects of PBNA exposure in man are not known. An epidemiologic study of exposed rubber industry workers failed to show a significant excess risk of bladder tumors; however, the results were not conclusive. A significant increase in the incidence of hepatomas has been observed in mice fed PBNA for 18 months. 2NNA, a byproduct in the production of alpha- naphthylamine, was shown to induce bladder tumors in dogs. In addition, in monkeys 2NNA was metabolized to BNA. General recommendations for handling substances that can be metabolized to carcinogens are given. These include treating substances that can be metabolized to carcinogens in the same way as carcinogens themselves and carefully evaluating the metabolic pathways of chemical agents used in the workplace. Specific recommendations for reducing exposure to PBNA are provided. These include using walk/in hoods, local exhaust ventilation, and other engineering controls, wearing protective clothing, and performing medical monitoring procedures that include examinations to detect the presence of bladder tumors and assays for BNA.