A new preparation of serum immunoglobulin G for intravenous use (IGIV) was investigated. The reagent, IGIV pH 4.25, is a liquid preparation of native, unmodified human serum IgG without preservatives but stabilized in 10% maltose. Its physical characteristics have multiple advantages over those of other available IGIV preparations. In a multicenter, randomized, double-blind, crossover study, this product was compared with a commercially available standard preparation, IGIV pH 6.8. Thirty-nine patients with various forms of primary immunodeficiency disease received infusions consisting of 400 mg/kg every four weeks; an overall total of 232 infusions were administered. Extensive clinical and laboratory observations were made. Adverse effects from infusion therapy were transient and minimal. No clinically significant abnormalities in vital signs and laboratory values occurred. There were no statistically significant differences between the results for IGIV pH 6.8 and those for the new IGIV pH 4.25 preparation. It was possible to infuse the new IGIV pH 4.25 reagent rapidly (0.1 ml/kg per hr) without inducing vasomotor adverse effects.