Sera from 114 breast cancer patients, 53 patients with other tumors, and 29 healthy controls were tested for antibodies reactive in antibody-dependent cellular cytotoxicity tests against a new breast cancer cell line (PMC9), two well-documented cell lines (MCF-7 and BT-20), and two melanoma cell lines. Of the breast cancer sera, 47% reacted with PMC9, whereas 30% reacted with the melanoma cell lines (P less than 0.05). Only 22% of sera from other cancer patients and 21% of sera from healthy controls reacted with PMC9 (P less than 0.05). The reactivity of sera from breast cancer patients was related to clinical stage of disease. Absorption studies on sera showing reactivity to both PMC9 and melanoma cells showed that the antimelanoma reactivity could be removed leaving anti-PMC9 reactivity intact. This study demonstrated the presence of breast cancer-associated antibody to PMC9 but not MCF-7 or BT-20 in the sera of breast cancer patients.