Commercially available immunoglobulin products suitable for intravenous application in humans are made by enzymatic or chemical modification of the IgG molecule. In order to examine, to which extent in vitro biologic functions of the IgG molecule are preserved in such preparations, specific antipseudomonal IgG from the rabbit was modified according to some of these procedures. The opsonizing activity of the different IgG preparations was evaluated in an in vitro system measuring the phagocytosis of Pseudomonas aeruginosa by rabbit granulocytes. The results demonstrated that the product Fab/Fc, which is made by papain or plasmin degradation of the IgG molecule, was still able to enhance phagocytosis but not to activate complement. The smaller papain- or plasmin-derived fragments Fab and Fc had no opsonizing activity. The pepsin-derived product F(ab')2 which possesses a divalent antigen binding site but lacks the Fc part, only enhanced phagocytosis when complement concentrations of more than 20% were present in the phagocytic system. Since the F(ab')2 fragment is not able to interact with Fc receptors or to activate complement via the classical pathway, the phagocytosis-enhancing activity of this molecule must be attributed to alternative complement pathway activation. In contrast to the enzymatically derived IgG preparations, a newly developed S-sulphonated IgG product was as efficient as unmodified IgG both in Fc- and complement-mediated opsonization.