Chromatography on controlled pore glass was adapted for the separation of immunoglobulin M (IgM) and immunoglobulin G (IgG) rubella antibodies from 0.3-ml samples of human serum. An extremely sharp separation of IgM from IgG antibodies could be obtained within 40 min. Nonspecific inhibitors were removed before chromatography by precipitation with high-molecular-weight dextran sulfate, and the titer of rubella antibodies in the different classes of immunoglobulins were assayed with a modified hemagglutination inhibition technique. The combination of these methods is recommended for routine tests. It permits an accurate diagnosis of recent rubella infection within a few hours.