P-glycoprotein (Pgp) has emerged as the central mediator in classic multidrug resistance in model systems in vitro. High levels of Pgp also have been detected in many normal human tissues and tumors; and its role in clinical drug resistance is currently under investigation. Recently significant levels of Pgp were localized to gravid and secretory endometrium; and it was demonstrated that the combination of estrogen and progesterone is sufficient to induce high levels of both Pgp mRNA and Pgp in uterine secretory epithelium. These findings suggest that increased Pgp expression also may be present in hormone-responsive malignancies such as endometrial adenocarcinoma. To determine whether Pgp is expressed in endometrial adenocarcinoma, 36 endometrial adenocarcinomas (grade I [n = 17]; grade II [n = 6]; grade III [n = 13]) were investigated retrospectively by the avidin-biotin-complex immunohistochemical procedure using three murine monoclonal antibodies (MAb) MAb C219, MAb C494, and MAb JSB-1, which recognize spatially distinct cytoplasmic epitopes of Pgp. Seventy-two percent of the tumors showed positive immunostaining with at least one MAb; 67% showed immunostaining with MAb C219, 50% with MAb C494, and 62% with MAb JSB-1. Forty-six percent of tumors were immunoreactive to two and 29% to all three antibodies. Membranous and Golgi/paranuclear type staining patterns were observed. Overall the intensity of immunostaining varied from one sample to another for a given tumor type, and considerable heterogeneity of expression was commonly seen within a given tumor. Strong to moderate immunoreactivity was seen in diffusely infiltrating, adenosquamous, and serous papillary carcinomas. In general, immunoreactivity to MAb C494 was weaker than MAb C219 or MAb JSB-1. Adenomatous and non-neoplastic endometrium adjacent to the tumors displayed strong membranous immunostaining with MAb JSB-1. Endometrial capillaries showed weak-to-moderate immunostaining to all three antibodies. It is concluded that Pgp is commonly expressed in endometrial adenocarcinoma and may be a significant factor responsible for their drug-resistant nature subject to modulation by progesterone.