Protein A (PA) is a cell wall component isolated from Staphylococcus aureus that binds specifically to the Fc region of IgG from several species including IgG subclasses 1, 2, and 4 from man. We investigated the effects of PA on the ability of rabbit IgG antibody to Forssman antigen to sensitize sheep erythrocytes (E) to lysis by excess guinea pig complement (GPC). The slope of the hemolytic dose response curve and the hemolytic titer of IgG were measured in buffer alone or in the presence of differing amounts of PA under conditions where PA had no significant effect on C activity. In buffer alone and at high ratios of PA:IgG the slope of the dose response curve was the same (4.4); but at high ratios of PA:IgG the titer of the IgG was markedly reduced. At lower ratios of PA:IgG the slope of the dose response curve was close to 1, a characteristic of IgM-like antibody.