Human sera from myeloma patients were subjected to two-dimensional electrophoresis. Heterogeneity in molecular weight and in isoelectric point of the myeloma proteins were demonstrated on the protein map. The patterns of five major immunoglobulin classes differed from each other, implying that the five classes of myeloma immu,oglobulin can be distinguished by examining the two-dimensional electrophoretic patterns. Sugar content analysis suggested that differences in sialic acid content may be one of the origins of the heterogeneity of myeloma proteins observed in isoelectric focusing.