Expression of carcinoembryonic Tn antigen studied with VVA-B4 and GSI-A4 lectins with the monoclonal antibody 83D4 and of T antigen with LDL and PNA lectins with the monoclonal antibody ZCMO4, were examined in 54 malignant or benign human breast tumors and for MCF-7, T47D and MCF-10A cell lines of human breast tumors origin. For breast tissues, positive membrane labelling with D-GalNAc alpha-O-ser/thr (Tn-antigen) specific lectins and 83D4 MAb occurred in benign cases indicating that modification of glycoconjugates may precede the cytologic anomalies. In fibroadenoma, fibrocystic dystrophy, ductal hyperplasia and grade I invasive ductal carcinomas, the binding sites for lectins and 83D4 MAb were essentially on the cell membrane with labelling of both apical and basolateral compartments. In grade II and III, the labelling involved the cytoplasma, and cell heterogeneity appeared. The disappearance of reactivity observed for a large proportion of cells at grade III may be due either to the loss of glycosyltransferase, or to the lack of synthesis of the protein back-bone. Invasive lobular carcinomas showed labelling both on apical membrane and the outermost part of the cytoplasm with a distinct cell polarity. Lectin receptors are present at the surface of metastatic cells, possibly related to their involvement in adhesion. In all cases, T or sialosyl-T antigens are present at the surface of tumors cells. All cell lines from breast tumors cultured in vitro were labelled with lectins and monoclonal antibodies. The simultaneous presence of Tn and T antigens on the cells, indicates that the expression of Tn antigen is due to a partial but non total deficiency in the beta-1- > 3 galactosyltransferase involved in T-antigen synthesis.