Mycoplasmas are useful models for biochemical studies of the mechanism of complement-mediated killing by antibodies to various membrane components. The purpose of this study was to determine the membrane antigens involved in immune killing of Acholeplasma laidlawii. Antibodies to A. laidlawii membrane total lipids, glycolipids, and phospholipids could be induced in rabbits after injection of reaggregates of the purified lipids with Mycoplasma hominis protein as the carrier. Killing of A. laidlawii membrane lipids were less effective than anti-membrane protein antisera in killing the organisms. Of the antisera to lipid components of A. laidlawii membranes, antiserum to phospholipids showed a more pronounced killing effect than antiserum to glycolipids. The antibodies to A. laidlawii in the rabbit antisera belong predominantly to the immunoglobulin G class of immunoglobulins. Double-diffusion tests in agar indicated that two immunologically reactive proteins are located on the membrane surface.