The serum disappearance, metabolic clearance and whole body catabolism of homologous immunoglobulin fragments were studied in rats. The rapid disappearance of Fab fragments from serum in normal animals was no longer present after nephrectomy. In contrast, the serum disappearance curve of Fc fragments was not altered by nephrectomy. The results of three different experiments, however, indicated that similar to Fab fragments, some Fc fragments underwent filtration and degradation in the kidneys. First, the amount of intact Fc fragments excreted in the first day after injection increased from 6% of the injected dose in normal rats to 17% in rats pretreated with sodium maleate. Secondly, nephrectomy eliminated the rapid phase of whole body catabolism of inected Fc fragments. Thirdly, auto-radiographic studies showed localization of Fc fragments in the renal proximal tubule cells in the first 3 hr after injection. An identical localization was seen with Fab fragments. These results support the conclusion that removal from circulation by glomerular filtration and subsequent reabsorption and degradation in proximal tubule cells represent the major mechanism for catabolism of Fab fragments. Although some Fc fragments undergo the same fate, most injected Fc fragments equilibrate with unknown sites where they are possibly bound and made unavailable for filtration in the kidneys.