Two different classes of rat antibodies have been shown to mediate the i.p. antigen-induced release of histamine. The heat-stable antibodies, associated with IgGa, involved in histamine release could be clearly differentiated from heat-labile homocytotropic antibodies. Antibodies associated with IgGa have been previously shown to mediate the antigen-induced release of SRS-A. It was not possible to dissociate the two biologic activities associated with IgGa antibodies by physical means.