ObjectiveTo investigate the expression and distribution of CD15s antigen in breast cancer and its relationship with carcinogenesis, progression and metastatic proclivity. MethodsCatalyzed signal amplification(CSA) immunohistochemical technique was used to detect the expression of CD15s antigen in breast cancer and in adjacent normal mucosa. Immunoelectromicroscopic ultrastructural localization of CD15s antigen labelled by colloidal gold was also bserved.ResultsThe positive rate of CD15s antigen expression in primary breast cancer was 79.8%(75/94). In adjacent normal mucosa (n=10) CD15s antigen showed weaker staining. The positive rate of CD15s antigen expression in grade Ⅱ-Ⅲ (87.3%) was notably higher than that in grade Ⅰ (69.2%, P＜0.05). In patients with lymph node metastasis, the positive rate of CD15s antigen expression was 90.2%, which was significantly higher than 67.4% in nodes with no metastasis (P＜0.05). CD15s antigen immunoreactivity was mainly localized in the border membrane of cytoplasm, endoplasmic reticulum, golgi complex and surrounding nuclear membrane in tumor tissue, and in the border membrane of cytoplasm in adjacent normal tissue. Conclusion CD15s antigen is a practical parameter for evaluating the degree of malignancy and lymphatic metastatic proclivity of breast cancer. It can provide a new pathway to investigate the carcinogenesis and progression of breast cancer.