The protective effect of four commercial human gammaglobulin batches (I-IV) in mice was studied using six different strains of group B streptococci (GBS): types Ia; Ib; II, R-protein negative (R-); II, R+; III, R-; and III, R+. Each mouse received 1.0 ml gammaglobulin and 0.5 ml bacteria, 10(6)-10(8) colony forming units (CFU). There was a close correlation between antibody levels measured by the use of radiolabeled protein A and the mouse-protective effect of the gamma-globulins. The mouse-protection tests demonstrated that batch I protected against GBS types Ia and III, R- at low concentration (65 mg/kg mouse weight), against type Ib at medium (260 mg/kg) and against type III, R+ at high concentration. Batch IV protected against types Ia and Ib, although the doses were four times higher than those in batch I, but did not protect against type III, R+. There was no mouse protection by any of the batches against type II. Antibody levels against Ibc and R, protein antigens, were substantially lower in batch IV. Because the results of these mouse-protection studies indicate the importance of such antibodies against protein antigens, batches I-III might be more useful for therapy of neonatal GBS-septicemia.