Specific cell membrane surface receptors for immunoglobulin and complement components were identified by rosette formation on in vitro cultured alveolar macrophages obtained from 24 humans and from rabbits. Respiratory macrophages have easily identified receptors for IgG and the C3b fragment of the third component of complement. A macrophage receptor for the C3d fragment was detected only when purified human complement components were used to form erythrocyte-antibody-C3 immune complexes but was not detected when whole human serum was used as the source of complement. No IgM cell receptor was identified. Thus, with respect to membrane receptors, alveolar macrophages resemble peripheral blood monocytes. These studies in addition emphasize the importance of using a variety of immune reagents, especially the use of human reagents, to determine correctly these cell receptors.