Infiltrating duct carcinomas were induced in the mammary glands of 7 out of 60 experimental female mice force-fed with the antifungal drug griseofulvin, at a daily dose lever of 0.3 mg/30g body weight, for 12 months. The first mammary gland tumor appeared 8 months after the initiation of feeding. Other pathological changes such as bronchocentric granulomas appeared in some male and female treated animals (16.5%).