Mouse mammary glands respond to carcinogen stimulus to form mammary lesions in organ culture. Prevention of the formation of mammary lesion has been utilized as a test to evaluate the effectiveness of a variety of classes of agents. In the present study we determined whether the effective chemopreventive agents are active against initiation or the promotion phase of lesion development. Mammary glands were subjected to 24 hours exposure to 2 mg/ml dimethylbenz (a) anthracene (DMBA) followed by a 5 day exposure to 7,12-tetradecanoyl phorbol - 13 - acetate (TPA). This treatment protocol allows one to study initiation and promotion aspects of lesion development. Chemopreventive agents effective when present prior to the carcinogen were considered as anti-initiators, whereas agents effective when present after the DMBA treatment along with TPA were considered as anti-promoters. Within the chemopreventive agents evaluated, limonene, oltipraz, aspirin, curcumin and b-sitosterol were anti-initiators. Esculatin, thiolutin, silymarin, DHEA and a few others were found to be anti-promoters. Results presented in this report can be utilized to study the efficacy of these agents in vivo.