The cellular localization of a 24,000 (24K) mol wt protein was evaluated by monoclonal antibody immunocytochemistry in atrophic and persistent proliferative endometrium as well as in various forms of hyperplastic and neoplastic human endometria. 24K was chiefly located in the cytoplasm of ciliated cells and their precursor forms, clear cells. It was absent in atrophic, resting, and regressive forms of cystic glandular hyperplasia. Persistent proliferative endometria with a low degree of proliferation has occasional 24K immunostained cells, whereas those with active proliferation contained intense 24K immunostaining. A similar pattern characterized the active forms of cystic glandular hyperplasia. The immunostaining reaction decreased in adenomatous and atypical adenomatous hyperplasia as well as in well differentiated adenocarcinomas. In poorly differentiated carcinomas, 24K protein was virtually absent. These findings suggest that 24K protein is estrogen and ciliated cell related, and endometrial ciliogenesis and 24K protein are morphological and biochemical markers, respectively, of the estrogenic endometrial response. Their increase and decrease in hyperplastic and neoplastic endometria support the concept that early hyperplasia is highly sensitive to estrogenic stimulation, whereas with increasing architectural and cytologic atypia, the estrogenic response decreases as a reflection of growing cell populations that are independent of estrogenic influence. Immunostaining with 24K is not sensitive enough to discriminate estrogen-independent cells of atypical adenomatous hyperplasia from those of early, well differentiated adenocarcinoma.