The glycolipid haptens of Mycoplasma pneumoniae were bound to membrane proteins of acholeplasma laidlawii (formerly Mycoplasma laidlawii ) by reaggregation. This process consisted of the solubilization of lipid-depleted A. laidlawii membranes and M. pneumoniae glycolipids in 20 mM sodium dodecyl sulfate, and dialysis of the mixed solutions against 20 mM Mg 2+ . The hybrid reaggregate, collected by centrifugation, was highly immunogenic in rabbits, eliciting the production of a high titer of antibodies that fixed complement with the purified glycolipids and inhibited the metabolism of M. pneumoniae cells. The free glycolipids, or their mixture with A. laidlawii proteins, were much less effective in stimulating these antibodies. The antibodies to the hybrid reaggregate agglutinated M. pneumoniae cells and inhibited their ability to absorb erythrocytes, which indicates that at least some of the serologically-active glycolipids are exposed at the outer membrane surface. The ability to bind selected membrane lipids to membrane proteins of a serologically unrelated species provides a new tool for producing antibodies to lipid haptens.