Three methods were compared for sensitivity and specificity in detecting rubella IgM antibody in early sera from post-natal infections and in sera from congenital infections. treatment pro-Mercaptoethanol duced a significant reduction in hemagglutination-inhibiting antibody titer in only 2 of 20 sera, but sucrose density centrifugation and indirect fluorescent antibody staining with an anti-IgM conjugate demonstrated rubella IgM antibody in all 20 sera. None of the methods detected rubella IgM antibody at a titer of 1:8 or greater in 10 sera from long-past infections. The anti-IgM conjugate appeared to be somewhat less specific than the anti-IgG conjugate from the same commercial source. It gave low FA staining titers with a number of 7S gradient fractions which could not be shown to contain IgM by immunodiffusion or immunoelectrophoresis; the anti-IgG conjugate did not stain 19S gradient fractions unless they were shown to be contaminated with IgG. Anti-IgM conjugates from 4 different commercial sources were found to vary widely in their ability to detect rubella IgM antibody. The sensitivity of FA staining for detection of IgM antibody was also affected by the time and temperature of incubation, and the specificity of sucrose density gradient centrifugation by the volume of fractions collected and the time of centrifugation.