Three not yet commercially available immunoglobulin preparations, intended for intravenous administration, were tested for their purity, physical integrity and in vitro functional activity. Preparations had been factory treated either at pH 4 and with insolubilized pepsin or at pH 4.25 only. Nephelometry revealed a high degree of isotypic purity (IgG greater than 99%). Gel filtration and SDS-PAGE analysis showed only trace amounts of aggregates and/or contaminants, particularly in one of the pepsin treated preparations. Protein A binding inhibition test and phagocytosis of Candida albicans by normal human polymorphonuclear leukocytes, after opsonization of the yeast with different concentrations of immunoglobulin preparations, showed the functional integrity of both the Fc and F(ab)2 regions in the three IgG preparations. Compared to previously reported methods of preparation which produce structural alterations of the Ig molecule, present data indicate the suitability of preparative methods aimed at preserving the integrity of the Ig molecule. Preparation and storage at pH 4.25 without any other treatment appear to be sufficient to obtain pure, aggregate free and in vitro functionally active Ig preparations.