A carcinogenicity study of nitrofurantoin formulated as Macrodantin was undertaken. The doses used were 0.0, 50.0, 100.0 and 200.0 mg/kg/day. Increased mortality was observed in male mice given 200 mg/kg/day. Chronic toxicity was observed in the kidneys of male mice: the normally occurring chronic nephropathy was somewhat increased in severity. The gonads of both male and female mice showed evidence of atrophy and degeneration. The ovaries showed an increased incidence of multilobular cysts but no evidence of neoplasia. A significantly higher incidence of malignant lymphoma in the top-dose males was offset by a non-significant difference in the opposite direction in females. Reasons are given for regarding this as a chance finding. The observations in this study indicate that the therapeutic uses of nitrofurantoin would not present a carcinogenic hazard to man.