This report describes a simple method for inactivating 19S red cell antibodies using the odorless compound dithiothreitol (DTT). 30 sera which contained antiglobulin-active red cell antibodies showed virtually no alteration in activity after DTT treatment, while 30 different sera containing cold-active red cell antibodies showed almost total elimination of activity following DTT treatment. Six sera were separated into IgG and IgM fractions. All red cell activity was eliminated in the IgM fraction, but was unaffected by DTT treatment of the IgG fraction. This method of inactivating IgM antibodies should be of value in investigating cases of hemolytic disease of the newborn and in studying sera containing mixtures of warm and cold red cell antibodies.