The ability of monoclonal mouse IgM, IgD and IgG anti-(4-hydroxy-3-nitrophenyl) acetyl (NP) antibodies to activate mouse complement was studied using a hemolytic assay. Efficient hemolysis was obtained with IgG2a, IgG2b and IgG3 antibodies but no lysis was observed using a monoclonal IgD. Of several IgG1 antibodies tested, three gave no detectable hemolysis, although weak but significant hemolysis was obtained with two other IgG1. IgM was found to be powerfully hemolytic in that it was effective at lower concentrations than were IgG. However, using complement from mouse strains carrying the H-2k haplotype it was found that, under conditions of complement limitation and saturating antibody, a fixed amount of complement could lyse about 3 to 4 times as many IgG-coated sheep red cells as IgM-coated red cells. This discrimination between IgM and IgG as regards the efficiency of complement utilization is controlled by a gene in the S region of H-2 and is not apparent with complement from mice carrying the b, d or s allele at that locus.