Serum samples from 10 infants surviving type III, group B streptococcal (GBS) meningitis were collected acutely and longitudinally for 6 months to determine patterns of antibody response to the capsular polysaccharide and their in vitro functional correlates. Five infants who failed to develop specific antibody at a mean of 3.8 weeks after diagnosis had an increase of greater than or equal to 1.0 microgram/ml after another 4-8 weeks. This IgM-predominant type-specific antibody declined to baseline 2-4 months later. Opsonophagocytosis of type III GBS increased from 0 to 88% in parallel with peak antibody response. Three infants developed increased antibody and opsonophagocytosis at 15-31 weeks after diagnosis, while two had no detectable response. Despite increasing complement levels, opsonophagocytosis of type III GBS was poor with low specific antibody levels These results suggest that survivors of GBS meningitis transiently develop specific antibody and associated efficient opsonophagocytosis, but functional competence does not persist despite maturation to adult levels of complement proteins.