Effects of the chronic ingestion of lycopene, a carotenoid from tomato, on the development of spontaneous mammary tumours were examined in a high mammary tumour strain of SHN virgin mice. Beginning at 40 days of age, the control and the experimental groups were allowed free access to an AIN-76TM diet and a diet supplemented further with lycopene at the concentration of 5.0 x 10(-5)%, respectively. The treatment significantly suppressed the mammary tumour development, which suppression was associated with the decrease in the mammary gland activity of thymidylate synthetase, and serum levels of free fatty acid and prolactin. Body weight was little affected and no deleterious side-effects of lycopene were detected. All results show that lycopene could be promising as a chemopreventive agent for mammary and other types of tumours.