The role of the complement system in host defense against Salmonella infection is poorly defined. Bacterial cell wall O-antigen polysaccharide can activate the alternative pathway in vitro. No studies, however, have elucidated the role of the classical pathway in immunity to Salmonella spp. in vivo. C1q-deficient mice (C1qa-1-) on a 129/Sv genetic background and strain-matched controls were infected intraperitoneally and intravenously with Salmonella enterica serovar Typhimurium and monitored over a 14-day period. After inoculation by either route, the C1qa-1- mice were found to be significantly more susceptible to Salmonella infection. Hepatic and splenic bacterial counts, performed at various time points, showed increased numbers of colonies in complement-deficient mice compared to controls. Analysis of blood clearance showed no difference between the two experimental groups during the first 15 min. However, after 20 min and until 6 h postinfection, numbers of circulating bacteria were significantly higher in complement-deficient mice. In vitro experiments using either resident or thioglycolate-elicited peritoneal macrophages showed a significant increase in the number of bacteria inside C1q-deficient macrophages compared to controls irrespective of the serum used for opsonizing the bacteria. These findings could not be explained either by an increased bacterial uptake, analyzed in vitro and in vivo using green fluorescent protein-tagged salmonellae, or by a defect in the respiratory burst or in NO production. The data presented here suggest the possibility of novel pathways by which C1q may modulate the pathogenesis of infectious diseases caused by intracellular pathogens.