The expression of Ley blood group antigen in epithelial ovarian cancer tissues and cell lines has been studied using a Ley-specific monoclonal antibody (MAb 3s 193). In ovarian cancer specimens, Lev was expressed in 75% of the 140 tumor specimens examined, with strong or moderate expression being observed in 56% of the samples. Seven of the I I ovarian cancer cell lines studied were Ley-positive. Using immunochemical approaches, Ley epitopes were found to be expressed on 4 types of carrier molecules: CA I25 ovarian cancer antigen, MUC-I mucins, lower m.w. glycoproteins and glycolipids. In cell lines, L e y was more commonly expressed on MUC-I much than on CA125, whereas in tumor specimens Ley was commonly found on both CAI25 and MUC-I. The biochemical nature of the smaller Lev glycoproteins was not determined, but it was shown that they were not CEA and LAMP-I, known Ley carriers in some other tumor types. Glycolipids carrying Ley epitopes were detected in both ovarian cancer cell lines and tumor specimens. The presence of Lev epitopes on a number of different molecular carriers, including 2 major ovarian cancer antigens (CAI 25 and MUC-I). explains the high incidence of Ley in ovarian cancer. The high expression of Ley in ovarian cancer and the availability of specific murine and humanized MAbs make Ley an attractive candidate target for clinical studies.