Human fetal tissues were studied for presence of immunoreactive Thomsen-Friedenreich (T) and Tn epitopes (EPs) using well-defined anti-T and anti-Tn rodent monoclonal antibodies. T and Tn are universal (pan) carcinoma (CA) markers that are occluded in normal postfetal tissues except in some immunoprivileged enclaves. Immunohistochemical methods using avidin-biotin immunoperoxidase for staining were employed. Tissues between 45 and 117 days after ovulation were studied. In most instances, anti-T and anti-Tn antibodies showed similar immunoreactivity as demonstrated by positive immunohistochemical staining. The most intense staining was in epithelial and mesothelial components; the mesenchyme stained more faintly. All human sera have anti-T and anti-Tn antibodies, stimulated largely by intestinal flora. The presence of immunoreactive T and Tn during an early phase of fetal development, as shown here, and their known absence in noncarcinomatous postfetal tissues suggests that T and Tn, in addition to their association with CA, are stage-specific oncofetal antigens in pretolerogenic differentiation phases.