The ability of concanavalin A (Con A) to inhibit the complement consumption by sheep erythrocytes coated with IgG, (IgG2-SpA1)2 complex and IgM anti-Forssman antibodies was studied. The (IgG2-SpA1)2 anti-Forssman antibody complex showed an increased hemolytic activity compared to uncomplexed IgG antibody. Con A in the presence of the (IgG2-SpA1)2 complex or the IgM antibody exhibited a high inhibitory capacity. An inhibitory capacity of Con A on IgG antibody hemolytic activity was also proved but at a Con A:IgG antibody concn ratio about 10 times higher than that for the Con A-IgM antibody system. A time dependence of Con A inhibitory capacity was determined, thus explaining the variable behaviour of the IgG antibody in the inhibition experiments. Radioactivity experiments demonstrated that only half of the Con A interacted with the (IgG2-SpA1)2 complex compared to the amount of Con A which interacted with IgG uncomplexed antibody.