Sera obtained from 103 normal individuals of blood group O, A, and B were tested in dialyzed and undialyzed 2‐mercaptoethanol reduction procedures. The same results were obtained with both technics with 94 per cent of anti‐A and/or anti‐B antibodies tested. In addition, identical results were obtained by both methods with 31 incomplete erythrocyte antibodies. The two reduction procedures had a similar range of sensitivity. It is suggested that dialysis is not necessary when 2‐mercaptoethanol reduction is employed to differentiate between IgG and IgM ant