CD24 is a small, mucin-type glycosylphosphatidylinositol-linked cell surface molecule expressed by neutrophils, pre B lymphocytes and certain human tumor cell lines. CD24 has been identified as a ligand for P-selectin in both mouse and human cells. We previously reported that the P-selectin-CD24 binding pathway is important for the binding of the breast carcinoma cell line KS to platelets and the rolling of these cells on endothelial P-selectin. In the present study we have analyzed the expression of CD24 on human breast carcinoma cell lines and on fresh breast carcinoma specimens using the CD24-specific antibody ML-5. Our study clearly demonstrates that CD24 is abundantly expressed on cell lines and fresh tissues of breast carcinomas. We find a differential expression of CD24 in breast carcinomas (cytoplasmic pattern) versus benign breast lesions (apical pattern). Moreover, the intensity of CD24 expression increases with the histological grade of the tumor. Thus, CD24 expression might be a useful marker for human breast carcinoma and play a role in facilitating metastasis by the interaction between tumor cells and platelets or endothelial cells.