IgM (IgG-depleted fraction of the serum) of donors of blood group A but not of donors of blood group B inhibits the reactivity of affinity-purified autologous anti-A IgG with A antigen. Affinity-purified anti-A IgG were obtained as described in the legend to Fig 3. Increasing concentrations of IgM (IgG-depleted fraction of the serum) of donors of blood group A1 (•) and of blood group B (□) were coincubated with affinity-purified, biotinylated anti-A IgG of individuals of blood group A1 before assessing the binding of IgG to A antigen by ELISA.