Induction of tumours of the urinary bladder in F344 rats by dietary administration of o-phenylphenol

o-Phenylphenol (OPP), a fungicide approved as a food additive in Japan, was given in pelleted diets at dietary levels of 0.156, 0.313, 0.625, 1.25 or 2.5% to groups of 11 or 12 F344/DuCrj rats of each sex for 13 wk, and 0.625, 1.25 or 2.5% to groups of 20-24 male F344/DuCrj rats for 91 wk. In the 13-wk study, transitional cell papillomas of the urinary bladder occurred in 6/12 (50%) of the male 1.25% group. In the 91-wk study, urinary bladder tumours appeared in 23/24 (96%) of the 1.25% and 4/23 (17%) of the 2.5% group. Among these tumours, transitional cell carcinomas were found in 20/23 (87%) of 1.25% and 2/4 (50%) of the 2.5% groups. A dose-related increase in the incidence of nephritic lesions was also observed in dosed rats in both the 13- and 91-wk study.