Sera from rubella patients were fractionated with six different gels on a 0.9 X 30 cm column. Fractionation efficiency was evaluated by determining the IgM, IgA and IgG concentrations in each fraction and titring the rubella virus hemagglutination-inhibiting antibodies before and after treatment with 2-mercaptoethanol. All six gels were able to separate IgM from the other Ig, but the distance between their elution peaks varied considerably. Assessment of the usefulness of a gel for diagnostic purposes requires evaluation of its ability to separate IgM from the oligometric IgA often present in sera and a possible source of diagnostic error. Ultrogel AcA 22 and Bio-Gel A5m gave the best results. In addition to displaying good peak separation, they eluted IgM within their linear fractionation range.