Intravenous infusions of 10 ml of normal human plasma per kilogram of body weight into children with severe deficits in humoral immunity resulted in serum IgG concentrations three weeks after treatment averaging 113 mg/100 ml higher than concentrations found in patients given 0.6 ml/kg of immune serum globulin (gamma globulin) intramuscularly, although both preparations provided approximately 100 mg/kg IgG (P <.0005). Low but higher than baseline concentrations of IgA and IgM could be detected after three weeks in the plasma-treated but not in immune serum globulin-treated patients. Three weeks after treatment, only one immune serum globulin-treated patient had detectable anti-B antibodies, none had antibodies to A cells or diphtheria toxoid, and all had low to absent titers of antitetanus antibodies; all plasma-treated patients had normal antibody titers to tetanus toxoid and low but detectable antidiphtheria antibodies, and one type O recipient of type O plasma had near-normal isohemagglutinin titers.