Sera from 31 patients with plasma cell myeloma and seven with macroglobulinemia were analyzed by the disc electrophoresis method (Ornstein and David). Of the 31 myeloma cases, 24 were found to have a single component. In the remaining cases, in spite of apparent homogeneity in paper electrophoresis, several abnormal components were seen. Ultracentrifugal studies revealed that the heterogeneous myeloma components are a group of globulins with a sedimentation rate between 7S and 19S. Some of the latter components could be easily dissociated by sulfhydryl liberating compounds. Comparison of the results with those obtained by paper and immunoelectrophoresis demonstrated that all the heterogeneous proteins were fast globulins of the β2A type. The possible significance of the heterogeneous myloma proteins is discussed.