Strains of type III group B streptococci isolated from patients with neonatal sepsis are generally resistant to complement-mediated phagocytic killing in the absence of specific antibody. It has been suggested that the resistance of type III group B streptococci to phagocytosis results from inhibition of alternative-complement-pathway activation by sialic acid residues of the type III polysaccharide. To better define the relationship between structural features of the type III capsule and resistance of type III group B streptococci to complement-mediated phagocytic killing, we measured deposition of human C3 on group B streptococcal strains with altered capsule phenotypes. C3 binding was quantified by incubating bacteria with purified human 125I-C3 in 10% serum. Wild-type group B Streptococcus sp. strain COH1 bound eightfold fewer C3 molecules than did either of two isogenic mutant strains, one expressing a sialic acid-deficient capsule and the other lacking capsule completely. Similar results were obtained when the incubation with 125I-C3 was performed in serum chelated with Mg-ethylene glycol-bis(β-aminoethyl ether)-N,N,N',N'-tetraacetic acid (MgEGTA), suggesting that the majority of C3 deposition occurred via the alternative pathway. In contrast to the wild-type strain, which was relatively resistant, both mutant strains were killed by human leukocytes in 10% serum with or without MgEGTA. We also measured C3 binding to 14 wild-type strains of type III group B streptococci expressing various amounts of capsule. Comparison of degree of encapsulation with C3 binding revealed a significant inverse correlation (r = -0.72; P < 0.01). C3 fragments released by methylamine treatment of wild-type strain COH1 were predominantly in the form of C3bi, while those released from the acapsular mutant were predominantly C3b and those from the asialo mutant represented approximately equal amounts of C3b and C3bi. We conclude from these studies that the sialylated type III capsular polysaccharide inhibits alternative-pathway activation, prevents C3 deposition on group B streptococci, and protects the organisms from phagocytic killing.