Previous studies have shown a reduction in mortality rate from 90% to zero when neonatal rats, inoculated with group B streptococci (GBS) were injected with type-specific IgM antibody. However, in those studies, the antibody was administered simultaneously with the bacteria and at the same site, unlike the situation which would exist if antibody was used clinically to treat established infection. In the present experiments, we administered antibody intraperitoneally at various intervals following intrathoracic inoculation of GBS. When antibody was administered immediately after, or up to 2 h following bacterial inoculation, all animals survived. When antibody administration was delayed for 4, 5, or 6 h, survival rates of 92, 60, and 29% were observed. When antibody administration was delayed for more than 6 h, no survival occurred. Failure of antibody to protect animals from death coincided temporally with profound depletion of the neutrophil storage pool. In other experiments, depletion of the neutrophil storage pool was produced by a separate, noninfectious mechanism (subcutaneous implantation of sterile polyvinyl sponge discs) after which animals were inoculated with GBS. Antibody did not provide protection from death in animals with neutrophil storage pool depletion.