The effect of sublytic doses of the complement membrane attack complexes (MAC)1 on protein synthesis in human leukemic cells was examined. As shown herein, rapid protein synthesis is evident in K562 erythroleukemic cells upon exposure to sublytic complement doses. Analysis of cell extracts by SDS-PAGE revealed high molecular weight proteins which appeared in the cells already after 15 min treatment with complement at 37°C, reaching a maximal level after 40-50 min. These large complement-induced proteins (L-CIP) were clearly observed in gels stained by Coomassie blue and in autoradiograms following [35S]-Met or [3H]-Leu incorporation. Rabbit antibodies prepared against L-CIP were reactive in immunoassays with extracts of MAC-treated cells but not of non treated cells. They also bound to the surface of intact K562 cells (as determined by immunofluorescence), but only after treatment of the cells with complement. Both heterologous (rabbit and guinea pig) and homologous (human) sera induced L-CIP synthesis. The induction of L-CIP was indeed mediated by the complement MAC since L-CIP could not be detected in K562 cells exposed to heat-inactivated human serum or C6-deficient rabbit serum. Similarly, C7- or C8-deficient human sera could not induce L-CIP production unless they were reconstituted with purified human C7 or C8, respectively. The synthesis of L-CIP was largely inhibited by the protein synthesis inhibitors cycloheximide and puromycin and partially inhibited by the RNA synthesis inhibitor actinomycin D. L-CIP was similarly induced in two other human leukemic cell lines, U937 and HL-60, but not in K562/S, a subline of K562 which is highly sensitive to complement damage. These results are discussed with respect to the resistance of leukemic cells, and nucleated cells in general, to complement-mediated immune damage.