Mouse and guinea pig macrophages cultured in vitro actively phagocytize non-opsonized thymocytes from certain heterologous animals including chickens, as shown in the accompanying paper (11). The present study was undertaken to investigate the mechanism of this phenomenon, using the phagocytosis of chicken thymocytes (c-thymocytes) by mouse and guinea pig macrophages. The involvement in this phenomenon of natural IgG passively adsorbed in situ to macrophages was excluded, since the phagocytosis of c-thymocytes was not significantly affected by the treatment of macrophages with homologous IgG or rabbit anti-sera directed toward homologous IgG. The involvement of lectin- or sugar-like receptors seems also to be unlikely, since various glycoproteins showed no significant effect. c-Thymocytes treated with normal mouse serum (NMS) but not with heat-inactivated NMS were strongly stained with goat anti-mouse C3 by an indirect immunofluorescent technique, and became extremely vulnerable to adherence to and phagocytosis by mouse macrophages, suggesting that c-thymocytes are an activator of the alternative pathway of mouse complement. These results as a whole raise the possibility that mouse and guinea pig macrophages can phagocytize c-thymocytes by recognizing their activating surfaces of the alternative complement pathway without the participation of exogenously added IgG or complement, as proposed by others in the phagocytosis of rabbit and mouse red blood cells by human monocytes.