Ovarian granulosa cells synthesize and secrete activin, a member of the transforming growth factor-beta (TGF-beta) peptide family, during the follicular phase of the menstrual cycle. We examined the growth-promoting activity of human recombinant activin-A on human luteinized preovulatory granulosa cells obtained from women undergoing in vitro fertilization. Activin-A induced proliferation of granulosa cells on day 5 of culture in a dose-dependent manner. Maximal effects were seen at concentrations greater than or equal to 100 ng/mL with an ED50 of 15 ng/mL. We suggest that activin-A is able to modulate growth of ovarian granulosa cells.