Abstract Large bowel cancer induced in rats by dimethylhydrazine (DMH) closely resembles human disease both histologically and clinically. In this study it has been used to assess the antimitotic drug razoxane (ICRF-159). Dimethylhydrazine induced benign colorectal tumours after 20 weeks and adenocarcinomas from 30 weeks. Razoxane was given from week 25 until the end of experiment (week 35) to see if this drug could inhibit the development of tumours. Animals were randomly allocated to four groups: DMH + razoxane (26), DMH + control (26), control + razoxane (25), control + control (25). There were fewer malignant colorectal tumours in rats receiving razoxane than in the controls (10 vs. 24; P = 0.04). Some animals developed malignant tumours of the small intestine as well as the large bowel and the number of malignant tumours for the whole intestinal tract was also reduced in animals receiving razoxane (15 vs. 32; P = 0.025). This study demonstrates inhibition of the development of malignant tumours in rats by razoxane. This finding may have relevance to colorectal cancer in man where razoxane has been used in disseminated disease and as an adjuvant to surgery.