Pooled human γ-globulin exhibited therapeutic activity in mouse infections induced with a variety of bacteria, with this action being related to the dose of γ-globulin. In order of decreasing susceptibility were strains of Staphylococcus aureus, Pseudomonas aeruginosa, Proteus vulgaris, Escherichia coli, Streptococcus salivarius, Streptococcus pyogenes and Diplococcus pneumoniae. Slight activity was noted against Salmonella typhimurium. The bacteria not affected by γ-globulin were Klebsiella pneumoniae, Pasteurella multocida and Streptococcus species, Group C. These findings appear to be consistent with the concept that normal antibodies are responsible for the described in vivo activity of γ-globulin.