The expression of the (alpha1-->3)-fucosyl-N-acetyl-lactosamine (CD15) epitope in the genital tract of the female rat during fetal and early postnatal ontogeny was investigated by means of immunohistochemistry. CD15 was exclusively associated with epithelial cells and was mainly located along the cell membrane. The CD15 expression was characterized, firstly, by considerable differences within the various structures and even substructures of the genital tract and secondly, by the high degree of time-related changes which accompanied the morphological development. In the Mullerian duct, CD15 was present from embryonic day (E) 14 until birth on the apical membranes throughout the epithelial cell layer. In the Wolffian duct, CD15 expression was present between E16 and E19. Along the longitudinal extent of the Wolffian duct, expression intensity differed, showing moderate to high levels in the epithelial cells of the cranial and caudal parts, but without recognizable CD15 expression in the intermediate part. In the urogenital sinus, CD15 was expressed from E15 until E21. In the cranial parts, all epithelial cells were positive, whereas in the caudal parts, CD15 was present only on their apical membranes. In the ovarian tube, uterine horn, and vagina, a moderate to high CD15 expression at birth gradually diminished to very low levels during postnatal days (P) 8 and 9. After P9, re-expression of CD15 occurred in the caudal part of the ovarian tube and in the uterus, increasing to a maximum at about P32. The findings provide (indirect) evidence for a correlation between the intensity of CD15 immunoreactivity and the serum concentrations of estrogens as well as of estrogen receptors in the urogenital tract. Since steroid hormone dependency can be regarded as a gauge of the differentiation of malignancies, it would be worthwhile correlating CD15 levels with those parameters.