A new test to determine the immunoglobulin class of erythrocyte antibodies is presented. Dithiothreitol (DTT), at a concentration of 0.01 m, has been used in a rapid 15 min, 37°C incubation test system. Dialysis is not required. The accuracy of this new DTT test was compared to a standard 2-ME procedure. Human sera containing saline-acting and incomplete varieties of erythrocyte antibodies were tested. Identical results were obtained by both methods with 292 of 302 (96.7%) test sera. Anion-exchange chromatography was employed to produce erythrocyte antibody solutions of known immunoglobulin class. Incubation of these solutions with DTT decreased the immunologic reactivity of IgM erythrocyte antibodies, without affecting the activity of IgG varieties.