The effects of sodium o-phenylphenate (OPP-Na) and o-phenylphenol (OPP) on 2-stage urinary bladder carcinogenesis in F344 rats initiated with N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) at levels of 0.01 or 0.05% in drinking water were investigated. Administration of 2.0% OPP-Na in the diet significantly increased both the incidence and the number per 10 cm of basement membrane of preneoplastic lesions (papillary or nodular hyperplasia; PN hyperplasia) of the urinary bladder in rats pretreated with 0.01% BBN, and also those of papilloma and cancer of the urinary bladder in the group pretreated with 0.05% BBN. Moreover, treatment with 2.0% OPP-Na, without prior BBN initiation, induced PN hyperplasia, papilloma and cancer. In contrast, OPP appeared to produce only a slight increase in the incidence of urinary bladder lesions following BBN, and its effect was not statistically significant. It also did not induce tumors of the urinary bladder. These results provide evidence that OPP-Na has promoting activity toward the urinary bladder while OPP does not. Moreover, the possibility that OPP-Na may be a complete carcinogen in the rat urinary bladder deserves consideration.