MUC1 is a cell-surface and secretory product of endometrial epithelium. Immunohistochemical studies carried out using two different antibodies to the mucin-type tandem repeat region of MUC1 indicate a cell-surface location in proliferative phase glands, with intracellular deposits accumulating in the early secretory phase. Commencing 3-4 days after the luteinizing hormone (LH) peak and continuing into the late secretory phase, secretory MUC1 appears in gland lumens. Uterine flushings were collected as a function of time after the LH peak and were analysed using a two-site enzyme-linked immunosorbent assay for MUC1. Low but measurable concentrations were observed up to day 7, while on days 7-13 much higher values were obtained. In women suffering from recurrent spontaneous miscarriage, the concentration of MUC1 in flushings was significantly lower than in the controls on day LH + 10. Lower values were observed on days 7 and 13. Reduced epithelial secretory function and a resultant change in uterine fluid composition are features of endometrium from recurrent miscarriage patients.