Prolonged papain digestion of rat IgG2a has recently been shown to yield two species of Fc fragments, termed Fc(I) AND Fc(II). The results of structural studies indicated that Fc(II) fragment was 15 to 20% smaller than Fc(I), probably secondary to loss of a carboxy-terminal peptide. The effects of these structural laterations on the catabolism and biologic properties of the Fc fragments were determined. The results of catabolic experiments indicated that after injection into normal rats Fc(I) fragments were retained in the circulation and slowly catabolized whereas Fc(II) fragments rapidly underwent filtration in the kidneys. In nephrectomized rats, however, both Fc(I) and Fc(II) fragments possessed identical slow rates of catabolism, as determined by serum disappearance and whole body catabolic experiments. Fragment pFc, corresponding to Cgamma3 domain, was different from either of the Fc fragments in exhibiting rapid rates of catabolism in both normal and nephrectomized rats. Fc(I) was more active in complement fixation and in adherence to the Fc receptor on monocytes in comparison with Fc(II). These results support the conclusion that catabolism of Fc and maintenance in circulation are separate processes influenced by different structures in the Fc fragment. The catabolism of Fc is controlled by structures in the Cgamma2 domain. This process probably is not related either to complement fixation or to adherence to the Fc receptor on monocytes. Some correlations between the structure and biologic properties of these Fc fragments are discussed.