A variety of antibodies detect the antigen defined by the cluster designation group 15 (CD15). These antibodies have been reported to be helpful in the diagnosis of many different diseases including leukemia, Hodgkin's disease, and non-Hodgkin's lymphoma, as well as aiding in the differential diagnosis between epithelial malignant mesothelioma and adenocarcinoma. In this review we describe the immunoreactivity of many of the CD15 antibodies, including MMA (Leu M1), anti-My1, C3D-1 (Dako-M1), VIM-D5, 1G10, MCS-1, anti-SSEA, and 3C4. The expression of the CD15 antigen in non-neoplastic human cells, leukemic cell lines and in neoplastic cells is reviewed.