Immunohistochemical study of the distribution of carbohydrate core-structures on O-linked glycoproteins (T, sialosyl-T, Tn and sialosyl-Tn) was performed using specific monoclonal antibodies on 148 primary breast lesions, including 10 normal breast tissues, 16 benign lesions and 122 invasive carcinomas (79 localized and 43 metastatic lesions). T antigen, not observed in normal breast tissue, was present in 31% of the benign lesions and in some cases of morphologically normal epithelium adjacent to tumor cells, compatible with altered glycosylation being an early event. Sialosyl-T (s-T) antigen was present in all cases of normal epithelium and in 81% of the benign lesions. Both Tn and sialosyl-Tn (s-Tn) antigen were present in normal breast lesions. Both Tn and sialosyl-Tn (s-Tn) antigen were present in normal breast tissue (30%) and benign lesions (31% and 19%). In the malignant lesions, 20% were positive for T antigen, 82% for s-T antigen, 66% for Tn antigen and 22% for s-Tn antigen. The staining pattern was nearly identical for carcinomas with and without lymph node metastases. In conclusion, immunostaining for simple mucins does not permit a clear distinction between benign and malignant breast lesions.