The anti-carcinogenic effect of TAP-144-SR biodegradable microcapsules of copoly (DL-lactic/glycolic acid) copolymer containing a potent LHRH agonist, TAP-144 (D-Leu6-(des-Gly10-NH2)-LHRH ethylamide, leuprolide acetate) was investigated in 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary tumours. At 50 days of age, all rats were given 20 mg doses of DMBA by gastric intubation. Rats on the Initiation Group were given a single s.c. injection of TAP-144-SR (3 mg/kg) 1 week before DMBA treatment, and rats in the Promotion Group were given repeated s.c. injections every fourth week from 1 week after DMBA treatment until the end of the experiment. In the Initiation Group, tumour incidence and multiplicity were reduced to 20.8% and 15.0% of the Control Group values, respectively. In the Promotion Group, tumour incidence and multiplicity were also reduced to 17.7% and 7.9%, respectively. It was suggested that TAP-144-SR may be a potential candidate as an agent for hormonal chemoprevention in breast cancer.