The expression of inhibin, a 10.7 kD follicle-stimulating hormone (FSH)-suppressing prostatic peptide of 94 amino acids, was investigated in normal human endometrium, endometrial hyperplasia, and adenocarcinoma, employing the avidin-biotin immunoperoxidase technique. The antiserum used was raised in rabbits against prostatic inhibin isolated from human seminal plasma. The study included 15 well differentiated, 32 moderately differentiated, and 21 poorly differentiated endometrial adenocarcinomas; 26 simple, five complex, and two complex atypical endometrial hyperplasias; and, for comparison, 25 normal proliferative and 30 normal secretory endometria. In malignant and hyperplastic endometrial tissues, inhibin was localized in the epithelial cytoplasm of endometrial glands while the stroma showed weak reactivity. On the other hand, inhibin was undetectable in the early proliferative phase, but was present on the luminal border of the glandular epithelium in the mid- and late proliferative phases. Secretory endometrium displayed strong inhibin reactivity in the cytoplasm of glandular epithelium and in the stroma. The increased inhibin reactivity in secretory endometrium as compared with the proliferative phase is indicative of a functional role for inhibin in the uterus. In addition, its localization in proliferative, hyperplastic, and malignant endometria suggests a possible regulatory role for inhibin in endometrial proliferation and growth.