Dithiothreitol (DTT) and 2-mercaptoethanol (2-ME) are sulfhydryl compounds that can be used to treat serum or plasma to denature IgM antibody reactivity. By using sulfhydryl agents, IgG and IgM antibodies can be separated, the relative amount of IgM and IgG antibodies can be determined, and the risk of hemolytic disease of the fetus and newborn can be assessed.