BACKGROUND Normal human cells resist the lytic activity of homologous complement (C) by expressing inhibitory molecules on their cell membranes. Recently, it has become increasingly evident that information on C inhibitors on malignant tumor cells is crucial before considering any immunotherapeutic attempts with C-activating antibodies. As one of the most potent inhibitors of C lysis is protectin (CD59), we have examined its expression and function on human breast cancer cells. METHODS Immunofluorescence microscopy was used to detect protectin expression on solid breast tumor samples (N = 12). Using immunoaffinity chromatography, protectin was isolated from the membranes of cultured MCF7 and T47D breast cancer cells. The purified proteins were incorporated into heterologous cells to study their C inhibitory activities. The reactivity of tumor cell protectins with terminal C complexes was examined by sucrose density ultracentrifugation analysis. A chromium release assay was used to study the effects of protectin neutralization on the sensitivity of MCF7 and T47D cells to C-mediated cytotoxicity. RESULTS Protectin was found to be strongly expressed by all human breast cancer tumors examined. The affinity-purified protectins had a glycophosphoinositollipid anchor and migrated in sodium dodecyl sulfate-polyacrylamide gel electrophoresis as glycosylated smears of 19 to 25 kilodaltons. Protectin isolated from T47D cells bound to nascent C5b-9 complexes generated in human sera and inhibited C lysis of guinea pig erythrocytes when incorporated into their cell membranes. C-mediated killing of breast cancer cells could be significantly enhanced after treatment of the cells with F(ab')2 fragments of the anti-protectin monoclonal antibody YTH53.1. CONCLUSIONS Human breast cancer cells resist C membrane attack by expressing protectin on their cell membranes. Neutralization of protectin on the surface of the tumor cells increases their sensitivity to C lysis.