We have developed a mouse monoclonal antibody, LDS60, against a cycle-dependent antigen by immunising (MF1 x BALB/c)F1 mice with a human endometrial membrane preparation. In formalin fixed paraffin embedded sections, LDS60 identified an epithelial specific antigen which exhibited a specific pattern of expression during the menstrual cycle. It was only occasionally expressed during the proliferative phase. During the early-secretory phase, there was intracytoplasmic staining in about half of the glands examined. This was in the form of small microvesicles, either near the base of the cell or supranuclear. In the mid-secretory phase the same proportion of glands exhibited staining in the form of micro-vesicles that were noted to accumulate nearer to the cell apices. In the late-secretory phase, there was no intracytoplasmic staining and the antigen was localised to the luminal border of the glandular epithelium and some staining appeared within the gland lumen of approximately 20% of glands. It is also diffusely expressed in some mucous secreting cells in the tongue, stomach and colon, as well as lung pneumocytes. The antigen has a molecular weight of approximately 200 kDa as identified by immunoblotting. This antigen exhibits similarities to MUC-1 which is involved in uterine receptivity and could therefore have a similar role. Its cycle modulation suggests that it could be used to monitor the uterine response to steroids.