The proliferative and differentiating effects of the gonadal hormones inhibin and activin-A were examined on cell lines derived from the ovary and testis. Activin-A was found to inhibit the growth of CHO-K1 (Chinese hamster ovary) cells in culture, with an IC50 of 3 ng/ml. The maximal response (50% inhibition) required 3 days of incubation in the presence of 40 ng/ml activin-A, and the inhibitory effect was accompanied by morphological changes. Inhibin (10 ng/ml) partially blocked the inhibition of growth by activin. Transforming growth factor-beta (TGF beta), which is structurally related to activin and inhibin, was a very potent inhibitor of the proliferation of CHO-K1 cells, with an IC50 of 0.2 ng/ml and a maximal effect (70% inhibition) at 2 ng/ml. The combination of high concentrations of both TGF beta and activin-A did not result in a greater inhibitory effect than that observed with TGF beta alone, suggesting an overlapping step in the mechanism of action for both factors. In contrast to the results with CHO-K1 cells, differential effects of activin-A and TGF beta were observed in R2C (rat Leydig cell testicular tumor) cells. Activin-A had only a slight effect on proliferation over a 4-day incubation, but inhibited progesterone accumulation ina concentration-dependent fashion within 12 h. TGF beta, on the other hand, was a potent inhibitor of both growth and steroidogenesis in R2C cells. These studies suggest that activin-A and inhibin may regulate proliferation as well as functions of gonadal cells.