The glycolipid haptens of Mycoplasma pneumoniae became immunogenic when bound to membrane proteins of Acholeplasma laidlawii by reaggregation. This process consisted of the solubilization of lipid-depleted A. laidlawii membranes and M. pneumoniae glycolipids in 20 m m sodium dodecyl sulfate and dialysis of the mixed solutions against 20 m m Mg 2+ . The antibodies produced in rabbits to the reaggregated glycolipids inhibited the metabolism of M. pneumoniae , fixed complement with M. pneumoniae glycolipids or whole cells, precipitated M. pneumoniae glycolipids, and agglutinated M. pneumoniae cells. All these antibody activities could be blocked or absorbed by the purified glycolipids but not by a series of carbohydrates containing glucose and galactose. It was concluded that the antiserum to the reaggregated glycolipids may be regarded as a specific serum to membrane glycolipids of M. pneumoniae , since the antibodies to A. laidlawii membrane proteins, present in this serum, did not react with the glycolipids or with any other cell component of M. pneumoniae.