Ninety macroglobulinaemia sera were tested for occurrence of multiple monoclonal immunoglobulins. Thirteen (14%) of the sera contained more than one monoclonal immunoglobulin. The homogeneous macroglobulins were isolated from 3 sera; in each case individually specific antigenic determinants common to the two components were demonstrated, in addition to determinants unique for each component. NH2‐terminal amino acid sequence studies in two patients showed that the IgM heterogeneity was due to differences in the primary structure of the variable parts of the μ (case Tö) and the μ (case æ) chains. Macroglobulinaemia appears to be a unique source of homogeneous immunoglobulins from related c