The opsonophagocytic activity of the four human IgG subclasses was studied using chimeric mouse-human antibodies with specificity for the hapten NIP. As target cells we used haptenized sheep red blood cells and N. meningitidis, labelled with different amounts of hapten. We used polymorphonuclear leucocytes (PMN) as effector cells to measure respiratory burst (RB), and U937 to measure phagocytosis/rosette formation. When the target cells were opsonized with antibody only, and PMN used as effector cells, IgG3 was highly efficient, while IgG1 revealed an intermediate activity and IgG2 and IgG4 were negative. The same pattern among the subclasses was obtained in the presence of complement source, when target cells with low hapten concentration were used. However, at high epitope concentration on the target cells, in the presence of complement source, IgG2 was highly active, while IgG4 was still negative or only slightly positive. When U937 were used as effector cells and complement was omitted, IgG1, IgG3 and IgG4 all revealed high phagocytic/rosette-forming activity, while IgG2 was negative. When the target cells were opsonized with antibody and complement, the phagocytic/rosette-forming activity was often suppressed. Our results reveal that all four human IgG subclasses possess opsonophagocytic capacity, but with different requirements concerning complement and Fc gamma Rs. They also enlighten us as to how IgG2 might perform its protective effect against harmful bacteria displaying high density of carbohydrate epitopes on their outside surface.