The relative complement-mediated lytic capability of the IgG subclass isotypes was studied using a matched set of mouse-human chimeric anti-(4-hydroxy-3-nitrophenyl)acetyl (NP) antibodies. The subclass pattern was shown to be highly dependent on variations in antigen concentration and to lesser extent on variation in epitope patchiness, antibody binding affinity and complement concentration. In general, the IgG3 subclass was most effective in inducing cytolysis at the different conditions used and only at high antigen concentration did the IgG1 subclass mediated more efficient cytolysis than IgG3. The IgG2 isotype required a relative high antigen concentration to be cytolytic while the IgG4 isotype was not cytolytic at any of the conditions tested. These individual characters of each of the IgG subclasses makes it conceivable that a subtle system of immunoregulation exists among the subclasses.