Paraffin-embedded tissue sections of primary tumors and lymph node metastases of 80 breast cancer patients were tested for the expression of Thomsen-Friedenreich (TF) antigens with the aid of a monoclonal IgM antibody (49H8) highly specific for phenyl-beta-galactoside. TF antigens were not expressed in 16 different normal tissues with the exception of some structures in the kidney. In tumor cells, two types of antigen expression were found; namely, cryptic and exposed. From stage T1/No to stages T2-4/N1,2 the number of cases expressing high amounts of TF antigens increased from 9% (2/22) to 22% (4/18) while the percentage of patients with low intensity of antibody binding was reduced from 59% (13/22) to 39% (7/18). The total amount of TF-positive primary tumors at stages T2-4/No increased from 42% (8/19) to 69% (18/26) when lymph nodes were infiltrated (T2-4/N1,2). At this stage 80% (21/26) of the patients with lymph node infiltration carried TF antigens in the nodes. The distribution of antigens was heterogeneous among the tumor cells and was expressed mainly in an apical or luminal position. The increased expression of antigens was attributed to exposed TF antigens, while cryptic antigens remained constant. When primary tumors expressed exposed TF antigens, the corresponding lymph nodes also contained exposed antigen. The same was true for the cryptic antigen. The data demonstrate an increase in the intensity of TF antigen expression during tumor progression and a spread of TF-positive tumor cells into the axillary lymph nodes with an increasing number of breast cancer patients being TF-positive at this stage.