Human endometrium is a glandular epithelial tissue with a substantial underlying stroma. Under the influence of ovarian steroids, endometrium undergoes a cyclical pattern of proliferation followed by secretory differentiation. Since retinoids promote the differentiation of many epithelia to secretory phenotypes they may be involved in controlling the secretory differentiation of human endometrial epithelium. Cytosolic binding proteins for retinol (cellular retinol binding protein) and retinoic acid (cellular retinoic acid binding protein) may play an important part in regulating the availability of retinoic acid to its nuclear receptors and we have therefore asked whether expression of mRNA for these proteins varies in relation to endometrial differentiation. In a series of 54 endometrial biopsies, both endometrial epithelial and stromal cells expressed mRNA for cellular retinol binding protein type I at a constant level throughout the menstrual cycle. Cellular retinoic acid binding protein type II was also expressed but the level of expression varied dramatically, being elevated in the proliferative phase and depressed during the secretory phase of the menstrual cycle in both epithelial and stromal cells. These data suggest that cytosolic binding proteins modulate the supply of retinoic acid to the nuclei of endometrial cells during the menstrual cycle and that retinoic acid is involved in the cyclical control of endometrial differentiation.