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The action of activin-A on Sertoli and spermatogonial cell proliferation during early postnatal life was studied by using in vitro organ culture of testis fragments from 9-day-old rats. Activin significantly stimulated 3H-thymidine incorporation into testis fragments cultured for 3 days in the presence of FSH, whereas it had no effect in the absence of the hormone. This effect was dose dependent in the range 10-200 ng/ml and was specifically inhibited by the activin-binding protein, follistatin. The effect of activin upon proliferation of different testicular cells was studied in detail by 5-bromo-2'-deoxyuridine-labeling fragments at the end of in vitro culture and then determining percentages of different labeled cells on immunostained histological sections. Concomitant treatment with FSH and activin, but not with FSH or activin alone, significantly stimulated Sertoli cell proliferation but markedly depressed that of differentiating type A spermatogonia. In contrast, proliferative activity of undifferentiated type A spermatogonia was independent of activin, irrespective of the presence of FSH. The effect of donor animal age was then investigated by culturing fragments derived from 3- and 18-day-old rats for 3 days. An age-related response was evident. Sertoli cell proliferation was stimulated by FSH alone in fragments from 3-day-old rats, activin having no apparent effect at this age. In contrast, none of the hormones tested either alone or in combination was effective in 18-day-old animals. These results demonstrate that activin acts with FSH in maintaining mitotic potentiality of Sertoli cells in a defined phase of their maturation path, when their proliferative activity is approaching the final arrest. These findings suggest that activin may be an important local factor in regulating Sertoli cell number and that the mitosis of differentiating spermatogonia subsides during Sertoli cell proliferation.