SummaryIn a 47 year old man with a IgG myeloma a ASL-O titer of 1: 62.500 T.U. was observed. After isolation of the M-protein from the other serum proteins, the monoclonal IgG fraction contained all the antistreptolysin activity. The ASL-O titer and the IgG concentration both decreased under the influence of a cytotoxic treatment. The ASL-O activity of the paraproteins was demonstrated with the hemolysis inhibition test, whereas with the Blue Aso test no passive agglutination was obtained. This discrepancy can be explained by the idiotypic Specificity of the monoclonal ASL-O activity.