An immunological study of 21 myeloma sera was carried out to determine their relationship to components of normal γ-globulin and to each other. Ten proteins were separated for detailed characterization. Every one of the myeloma proteins studied was immunologically different, indicating individual specificity. The γ-type myeloma proteins were all related to normal γ-globulin or a fraction thereof, and although each was distinct from the others, all were related. The ß-type myeloma proteins likewise were related to one another, though individually specific. Evidence was found favoring the view that some of the γ-type myeloma proteins are related to the ß-type myeloma proteins although most of them were completely unrelated. Likewise the results indicated a definite but distant relationship between the ß-myeloma proteins and normal γ-globulin. Certain of the ß-myeloma proteins were more closely related to a fraction of normal serum of similar electrophoretic mobility. It is suggested that most multiple myeloma proteins fall into two groups or families of cross-reacting abnormal proteins, one closely related to normal γ-globulin, the other distantly related to γ-globulin with marked individual specificities.