In this report we describe the expression of the adenocarcinoma associated antigen recognized by the monoclonal antibody 44-3A6, in various tissues during normal human fetal development. Conventional, formalin-fixed and paraffin-embedded sections of normal organs were examined from fetuses ranging from 9 to 42 weeks of gestation. Immunohistochemical localization of antigen-antibody complexes was accomplished using the avidin-biotin complex (ABC) method using horseradish peroxidase. The monoclonal antibody (MAb) 44-3A6 detects a cell surface 40 kD protein which is frequently expressed by adenocarcinomas and by select normal glandular tissues. Detectable expression of this protein was seen at different time periods during fetal development depending on the tissue. This expression was confined to a relatively small range of cell types and tissues; immunostaining was noted in select epithelial cells of the aerodigestive tract, exocrine pancreas, neural tissues, renal tubules, and transitional urothelium, as well as in other tissues. This immunostaining generally, but not invariably, corresponded with patterns previously reported in benign and/or malignant neoplasms of adult tissues. In most instances, once expression occurred within a tissue, it continued through gestation. These data show that this tumor associated gene product is differentially expressed in a broad range of normal developing human fetal tissues.