Differences in the O-antigen polysaccharide structure of lipopolysaccharide were previously shown to affect the rate of phagocytosis of Salmonellae strains by the murine macrophage-like cell line J774. Phagocytosis required a serum factor(s) that is labile to heat (56 degrees C for 30 min) and to zymosan treatment, which indirectly suggested the participation of C. We now show, using guinea pig serum, that these bacteria activate C3 at different rates, and this activation is proportional to the later rate of phagocytosis. Activation is predominantly via the alternative pathway, because C4 is not consumed and the reaction proceeds equally well in the serum of C4-deficient guinea pigs. Because the extent of activation of C3 and the subsequent rate of phagocytosis are inversely proportional to virulence, we propose that virulence of a strain may be influenced by the ability of the polysaccharide structure of its lipopolysaccharide to activate the alternative pathway of C, destining it for subsequent phagocytosis.