A modified form of serum immunoglobulin G (pH 4.25) was tested for its effect on neutrophil kinetics and survival rates in neonatal rats with type III, group B streptococcal pneumonia and sepsis. Each of 30 animals received a transthoracic inoculation of 105 organisms/g of body weight; all died within 48 hr. When 100, 1,000, or 2,000 mg of immunoglobulin G/kg was administered intraperitonally at the time of bacterial inoculation, survival rates rose to 20%, 90%, and 100%,respectively. Evenwhen the immunoglobulin preparation was administered intraperitoneally 2 hr after transthoracic inoculation of bacteria, all 19 animals survived.Only sevenof 15 animals survived when immunoglobulin administration was delayed for 22 hr. Immunoglobulin facilitated the neutrophil inflammatory response: when immunoglobulin (rather than an albumin control) was administered, neutrophils werereleased more rapidly from the storage pool and accumulated more quickly at the site of bacterial inoculation. Unlike infected control animals, immunoglobulin recipients did not develop neutropenia or depletion of the neutrophil storage pool.