Direct inhibitory effects of a gonadotropin-releasing hormone agonist (GhRh agonist, goserelin, GOS) were studied on the in vitro clonogenic growth of human breast cancer, in comparison with estradiol-17 beta (E2) and tamoxifen (TAM). Estrogen (ER) and progesterone receptors (PgR) were asseyed in the adjacent cancer tissues using the DDC method. In 19% or 12 out of 64 cancers, E2, 10(-8) M, was shown to be effective in increasing the number of colonies per dish to 150% or more of the control. On the other hand, 16% (10/64) responded to TAM or GOS, respectively, i.e. the plating efficiency decreased to 50% or less of the control. While E2 and TAM had a tendency to have better effects in ER-positive tumors than ER-negative ones, the inhibitory effect of GOS was not dependent on the presence or absence of ER or PgR in the tumor. The GOS sensitivity did not correlate with the E2 dependence or the TAM sensitivity. These results suggest that a possible direct action of GOS may not be mediated by the ER system.