The monoclonal antibody (MAb) B72.3 recognizes a mucin-like glycoprotein, TAG-72, which has been detected in a spectrum of human carcinomas, but not in the normal tissue counterparts. Using avidin-biotin-peroxidase complex (ABC) immunohistochemical techniques, MAb B72.3 was reacted with formalin-fixed, paraffin-embedded fetal and pediatric tissue sections to determine the extent of expression of the recognized antigen in these tissues. First trimester fetal tissues failed to express detectable antigen. Gastrointestinal epithelia from 13 to 34 weeks gestation demonstrated the most immunoreactivity with B72.3 although bronchial respiratory epithelium of the lung, transitional epithelium from the kidney, Hassall's corpuscles of the thymus, and gonadal tissues from fetuses of both sexes were also reactive. The TAG-72 antigen was not detected in fetal breast, pancreas, liver, spleen, adrenal, or heart. Expression of the TAG-72 antigen in malignancy appears to correlate well with fetal tissue reactivity with B72.3.