The role of complement receptor type 3 (CR3) in nonopsonic recognition of group B streptococci (GBS) by macrophages was investigated. Monoclonal anti-CR3 (anti-Mac-1) inhibited phagocytosis of GBS strains by as much as 50% in serum-free cultures of both mouse peritoneal macrophages and the macrophage cell line PU5-1.8. GBS uptake was unaffected by the presence of anti-C3 or salicylhydroxamate, an inhibitor of the covalent binding reaction of C3. Soluble antibodies to LFA-1 or to the common beta-chain (CD18) of the LFA-1/CR3/p150,95 family of cell adhesion molecules did not inhibit GBS uptake. Down-modulation of surface Mac-1 on macrophages following adherence to anti-Mac-1- or anti-CD18-coated surfaces also inhibited uptake of GBS. Further evidence for GBS interaction with CR3 was demonstrated by reduction of EC3bi rosette formation in macrophages adherent to GBS-coated plates. These studies suggest that GBS can interact with macrophage CR3, promoting phagocytosis in a C3-independent fashion. In the absence of specific immunity in neonates, this recognition mechanism may be a significant virulence determinant for GBS which poorly activate the alternate complement pathway.