Ninety macroglobulinaemia sera were tested for occurrence of multiple mono clonal 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 pa tients showed that the IgM heterogeneity was due to differences in the primary structure of the variable parts of the x (case To) and the μ (case Nae) chains. Macroglobulinaemia appears to be a unique source of homogeneous immuno‐globulins from related cl