Red blood cells of individuals of blood group A1 were incubated with affinity-purified anti-A IgG and anti-A IgM of individuals of blood groups A1 and B for 45 minutes at 37°C. The Coombs test was performed as described in the legend to Table 1. Inputs of anti-A antibodies were as follows: IgM from A1 individuals: 1.50 mg/mL; IgG from A1 individuals: 3.75 mg/mL; IgM from B individuals: 0.1 mg/mL; IgG from B individuals: 0.25 mg/mL.