A convenient turbidimetric phagocytosis assay was applied for the functional comparison of various intravenous IgG preparations. Staphylococcus aureus (Oxford) was opsonized by the immunoglobulin samples in the presence of an IgG deficient serum as a source of complement. The opsonized bacteria were subjected to phagocytosis by neutrophil granulocytes isolated from healthy adults. The time course of phagocytosis was monitored by the decrease of light absorbance at 400 nm. Changes in light absorbance during a 15 min period of opsonophagocytosis (delta E(400)) were expressed as a percentage of delta E(400) obtained by a reference IgG preparation. The opsonizing effect of five commercially available i.v. IgG preparations was compared. Three different preparations containing whole, non-modified IgG molecules had a comparable opsonizing effect while a further one prepared by propiolacton modification displayed a reduced activity (52%) of the reference preparation, taken as 100%. A preparation consisting of IgG molecules without an Fc-region proved to be practically ineffective (8.7%).