The catabolism of 131 I and 125 I paired labelled Fc fragments of myeloma proteins and of H chain disease proteins of different heavy chain subclasses was studied in men and monkeys. In contrast to the previously demonstrated catabolic heterogeneity of intact γG immunoglobulins, the Fc fragments and H chain disease proteins of all subclasses tested were catabolized at a similar rate. These data suggest that structures not present on the Fc fragments are responsible for the faster turnover rate of γG 3 immunoglobulins and for the differences in half-lives of myeloma proteins within a given subclass.