Prolonged papain digestion of rat IgG2a produced two molecular species of Fc fragments, termed Fc(I) and Fc(II). Studies by gel filtration chromatography and polyacrylamide gel electrophoresis in SDS/urea indicated that the two subunit polypeptide chains in each Fc preparation were associated by non-covalent bonds only. By analytical ultracentrifugation Fc(I) was found to have a m.w. of 47,100 and a sedimentation coefficient of 4.08S. Fc(II) had a m.w. of 39,800 and a sedimentation coefficient of 3.83S. The m.w. for the subunit chains of Fc(I) and Fc(II) were 25,300 and 20,300, respectively, as determined by analytical ultracentrifugation under dissociating conditions. Calculation of the frictional coefficient ratios indicated that both Fc fragments possessed compact globular structures. The difference in size between these two Fc fragments probably was due to a loss of some carboxy-terminal residues in Fc(II). Both Fc fragments possessed nearly identical amino-terminal amino acid sequences. Papain cleavage occurred primarily between residues 233/234 and 234/235. The carbohydrate compositions of the two species of Fc fragments were similar. It was concluded that under acid and reducing conditions papain cleavage of rat IgG2a occurred to the carboxy-terminal side of the hinge region. Prolonged papain digestion led to secondary attack in the carboxy-terminal end of the CGAMMA3 domain at an unidentified site, or sites, producing a stable second species of Fc fragments.