Group B streptococci are the major cause of sepsis and fatal shock in neonates in the United States. Although a number of clinical features have been associated with enhanced severity of disease, the role of soluble immune complex formation in group B streptococcal infection has not been evaluated. We determined the frequency with which circulating immune complexes occurred in 16 infants with nonfatal type III, group B streptococcal meningitis, using an immunoglobulin-specific C1q enzyme immunoassay. Ten healthy, age-matched infants served as a control group. Elevated levels of immunoglobulin M (IgM)-containing immune complexes were present in the sera of four (25%) patients with group B streptococcal meningitis. Group B antigen was detected in precipitated IgM immune complexes from each of these four infants by competitive enzyme-linked immunosorbent assay. In addition, IgG-containing immune complexes were present in 56% of sick and 60% of control infants. Group B antigen was demonstrated in the serum of a sick neonate containing only IgG immune complexes but not in controls. Our findings indicate that a subset of infants with type III, group B streptococcal meningitis develop IgM immune complexes containing group B-specific antigen, and these may persist for up to 3 months in some patients.