Using immunocytochemistry, we report the occurrence of a 10.5-kDa inhibin in human breast tissue specimens obtained from normal, fibroadenoma and adenocarcinoma cases. The immunostaining for inhibin was confined to the cytoplasm of the epithelium and myoepithelium cells. Expression of inhibin increased in following order: normal (1+); adenocarcinoma and lobular carcinoma in situ (2+) and fibroadenoma (4+). Breast explants has the ability of denovo biosynthesis of inhibin in vitro. In view of the growth modulating regulatory properties of 10.5 kDa inhibin, our findings are suggestive of the potential role of inhibin in breast pathology.