Sodium o-phenylphenate (OPP-Na) was given at dietary levels of O (control), 0.5, 1.0 and 2.0% to groups of 50 male and 50 female mice for 96 wk, and all the animals were maintained without OPP-Na for a further 8 wk. Both sexes given 2% OPP-Na and females given 0.5% and 1% OPP-Na showed growth retardation. Serum alkaline phosphatase activity in OPP-Na treated females was significantly increased in a dose-related manner. There were no treatment-related effects on clinical signs, mortality, urinalyses, haematology or organ weights. The incidences of several non-neoplastic and neoplastic lesions achieved statistical significance but none was considered to be related to treatment. There were increased incidences of haemangiosarcomas of the liver in males given 1% and of hepatocellular carcinomas in 1 and 2% males, and haemangiomas and leiomyomas of the uterus, present in the controls, were absent or decreased in all treated females. Therefore, this study did not demonstrate any clear carcinogenic effect of OPP-Na on mice at dietary levels of up to 2%.