A cell-surface sialoglycoprotein (LEA.135) was identified using a monoclonal antibody that was generated by immunization of Balb/c mice with extracts of normal breast tissue following prior immune-tolerization with mammary carcinoma cell lines. LEA.135 is distinct from other known epithelial cell-associated antigens, including the family of mucins or keratins and epidermal growth factor receptor. Using immunohistochemical staining methods, LEA.135 expression was detected predominantly on the apical plasma membrane of normal and neoplastic mammary and extramammary epithelial cells in freshly frozen or formalin-fixed paraffin-embedded tissue sections. A retrospective study of 111 cases of lymph node-negative patients (TanyN0M0) with primary infiltrating ductal breast carcinoma, with a median follow-up of 7.9 years, was conducted. A comparison of overall survival (O.S.) was made of patients whose tumor cells exhibited reactivity with anti-LEA.135 antibody (O.S. 92.9 +/- 3.3% at 8 years), compared with those whose specimens showed the absence of LEA.135 expression (O.S. 68.3 +/- 10.8% at 8 years). A statistically significant univariant association between LEA.135 expression and O.S. was observed (logrank p < 0.001). In addition, in a subgroup of patients with histologically moderately differentiated tumors (N = 71), LEA.135-positive cases showed an improved O.S. (90.8 +/- 4.6% at 8 years; p < 0.001) compared with those who were LEA.135-negative (O.S. 55.6 +/- 13.6% at 8 years). The association remained statistically significant in a multivariable analysis after adjusting for histological grade, tumor size and age (p < 0.02). Thus, in this series of patients with lymph node-negative primary breast carcinoma, LEA.135 expression was associated with a significant decrease in the rate of recurrence and with an increase in overall survival, independent of tumor size, histologic grade, and patient's age. In contrast to the majority of other prognostic markers which predicts a worse biology, LEA.135 is a unique class of antigen whose expression indicates a lower aggressiveness of the tumor cells.