The presence and distribution of a protein with a mol. wt of 24,000 (24K) was determined in endometrial biopsies from regularly cycling women and in women with endometrial carcinoma. This protein, of as yet unknown function and originally found in a breast cancer cell line, was detected by immunohistochemistry using a monoclonal antibody. In regularly cycling women, the 24K protein began to appear in the glandular epithelium during the late proliferative phase and decreased after ovulation. In contrast, in the superficial epithelium, the strongest immunostaining was observed during the secretory phase. Superficial epithelial cells expressed maximal 24K immunoreactivity around day 21 of the cycle and it was clearly seen in the bulbous projections of the apical cytoplasm. These results suggest that the 24K protein may be a marker for hormonal events in the endometrium during the menstrual cycle. In endometrial carcinoma, 24K was correlated with low tumor histologic grade, few mitotic figures, few nucleoli and a low degree of nuclear pleomorphism. These data suggest that 24K may be a potential marker of tumor differentiation.