Inhibins (alpha and beta heterodimers) and activins (beta homodimers) are related peptides with opposing biologic action on gonadotropin regulation. They serve as components of the pituitary-gonadal feedback system. Although sex-cord stromal tumors can usually be distinguished from ovarian epithelial tumors or their metastases by morphology or by using antibodies against intermediate filaments, the diagnosis remains difficult in rare situations in such cases as sarcomatoid granulosa-theca cell tumors, ovarian small cell carcinomas, or soft-tissue sarcomas. A total of 28 sex cord-stromal tumors of the ovary and 43 non-sex cord-stromal tumors were immunohistochemically evaluated for the presence of alpha and beta subunits of inhibin and activin. For comparison, 10 normal adult gonads including seven ovaries with hilar regions and three testes also were examined. Immunoreactivity for both alpha and beta subunits of inhibin/activin was identified in both non-neoplastic and neoplastic granulosa, Sertoli, Leydig, hilar and luteinized theca cells, with the strongest immunoreactivity in Leydig and hilar cells. One of three Sertoli-Leydig cell tumors that showed a sarcomatoid growth pattern and one sex-cord tumor with annular tubules also were immunoreactive for both subunits. For non-sex cord stromal-derived ovarian tumors, alpha subunit immunoreactivity was negative in all but two of five ovarian mucinous tumors. Weak immunoreactivity for beta subunit was found in most ovarian surface epithelial carcinomas, two of four colonic, and one of three pancreatic carcinomas. No immunostaining was found in nonspecialized gonadal stromal or interstitial cells, thecal cells, germ cells, ovarian small cell carcinomas, carcinoid tumors, dysgerminomas, or leiomyosarcomas. Immunostaining of alpha subunit (inhibin alpha), but not of beta subunit could serve as a sex cord-stromal differentiation marker because alpha subunit alone is largely confined to sex cord-stromal lesions with the exception of some ovarian mucinous tumors. Further studies are needed to define the usefulness of this sex cord-stromal differentiation marker in the practice of surgical pathology. Coexpression of alpha and beta subunits in sex cord-stromal elements suggests that dimeric inhibin is expressed in these cells.