The distribution of keratan sulphate (KS) in normal endometrium has been studied by immunohistochemistry using monoclonal antibody 5D4. KS is present in association with glandular epithelium throughout the normal cycle. Production is hormonally regulated, as indicated by the significant increase observed in the secretory phase of the cycle. Intracellular immunoreactivity increases to a maximum 3 days after the luteinizing hormone (LH) peak. This is followed by an increase in secreted KS from 4 days after the LH peak, and essentially all endometrial glands showed immunoreactive deposits by 5 days after the LH peak. Beginning on day 7 after the LH peak, a fraction of glands was observed to have lost the epitope. Nevertheless, KS remained in a significant proportion of the gland lumens 9 days after the LH peak. The data suggest that analysis of secretory KS in women gives an index of hormonally regulated epithelial differentiation in the peri-implantation phase.