The contribution of mannose receptors on the cell surface of mouse peritoneal macrophages to the pro cess of liposome‐induced phagocytosis of immunoglobu lin G‐opsonized sheep red blood cells (SRBCs) through Fcγ receptor has been investigated. Fcγ receptor‐mediated phagocytosis of opsonized SRBCs was activated by modified α ₂‐macroglobulin, which was produced in the incubation mixture of α ₂‐macroglobulin and liposometreated splenic B cells. The phagocytosis was specifically inhibited by the addition of D‐mannose, and the inhibition was dependent on the D‐mannose concentration. The binding of modified α ₂‐macroglobulin to macro phages was also reduced by the addition of D‐mannose. The activation effect of modified α ₂‐macroglobulin was not inhibited when in the presence of α ₂‐macroglobulintrypsin and ‐methylamine complexes. In the presence of cydoheximide, activated phagocytosis was reduced to the control level. By Scatchard plot analysis of IgG binding studies, the number of Fcγ receptors of a macrophage had been increased to 4.6‐fold that of a control macrophage by treatment with modified α ₂‐macroglobulin. These findings suggest that macrophage mannose receptors are involved in activating the process of FCγ receptor‐mcdiatcd phagocytosis of opsonized SRBCs induced by modified a α ₂‐macro globulin. Lectins may participate in a signal transduction in macrophage activation by lipo somes, J, Lcukoc, Biol. 57: 687–691; 1995.