Because rates of morbidity and mortality due to newborn sepsis are unacceptably high, adjunctive therapies must be investigated. In the current studies, after healthy adults were immunized with type III group B streptococcal (III-GBS) capsular polysaccharide vaccine, serum was obtained from "vaccine-responders" from which a pooled human IgG preparation hyperimmune for III-GBS was prepared by ion-exchange column chromatography. This preparation, containing 549 micrograms/ml III-GBS antibody was very active functionally when evaluated against multiple III-GBS strains both in vitro in an opsonophagocytic assay using newborn sera and in a newborn rat model of III-GBS disease. The level of functional activity was dramatically higher than that of commercially available human IgG preparations for intravenous use demonstrated previously in identical assays. Human IgG preparations hyperimmune for GBS offer promise for use as adjunctive therapy of sepsis in newborns.