A receptor binding to the C1q subcomponent of complement has been reported by many workers. In this paper we report for the first time that C1q receptor binds not only to C1q, but also to three other structurally similar ligands, namely mannan binding protein (MBP), conglutinin, and lung surfactant protein (SP-A). All these ligands have been reported to enhance removal of species bound to their globular domain from blood (MBP, conglutinin, C1q) or lung (SP-A) through phagocytosis. One of the possible roles for ligand-receptor binding may be initiation of phagocytosis.