Biphenyl and its derivatives, 2-aminobiphenyl, 2-nitrobiphenyl, 4-aminobiphenyl, 4-nitrobiphenyl, p-phenylphenol, o-phenylphenol (OPP), and o-phenylphenol sodium salt tetrahydrate (OPP-Na) were examined for their bladder carcinogenicity in rats by a short-term assay for agglutinability of bladder epithelial cells with concanavalin A. Increased agglutinability was observed after 1-week treatment with 2.0% and 1.0% OPP, 2.0% and 1.0% OPP-Na, 0.5 and 0.1% 4-aminobiphenyl, and 0.5% 4-nitrobiphenyl in the diet, suggesting carcinogenicity of these compounds in the bladder. Such an increase in agglutinability was not observed in rats fed diets containing biphenyl, 2-aminobiphenyl, 2-nitrobiphenyl or p-phenylphenol at 2.0%. With OPP-Na, an in vivo carcinogenesis experiment was performed. Bladder carcinomas developed in 14 of 36 male Fisher rats fed a 2% OPP-Na diet for 50 weeks.