Human serum containing antibodies for Mycoplasma pneumoniae or hyperimmune rabbit serum contianing antibodies for membrane glycolipids of M. pneumoniae sensitized M. pneumoniae to the lytic action of fresh guinea pig serum, which served as a source of complement. Lysis of the orgaisms was indicated by the rapid loss from the cells of 260 nm-absorbing materials, by a rapid decrease in viability, and by the transformation of the highly filamentous organisms to ghost-like structures, which was visualized by electron microscopy. The ghost membranes contained large irregular holes that probably resulted from osmotic lysis of the organisms, triggered by the damage to membrane permeability induced by the immune reaction.