Abstract.Using ultracentrifugation, gel filtration, and ion exchange, an attempt was made to completely separate immunoglobulin IgM and IgG present in pooled human serum of blood group O. The quality of separation was studied by testing IgG fractions for the presence of IgM by a sensitive detection method. It is demonstrated by rheophoresis that small amounts of IgM always remain detectable in the IgG fraction. This is of clinical importance because conclusions on the presence of specific antibody of different Ig classes are usually drawn under the assumption of complete separation. The control of the efficacy of the separation process using a sensitive detection method for Ig of different classes is recommended.