Reaction of S-carboxymethyl-Fc fragment from rabbit immunoglobulin G with cyanogen bromide cleaves the fragment into five unique peptides, each of which has been isolated by chromatographic methods. The amino acid composition of the five peptides accounts closely for the amino acid composition of the Fc fragment. On the basis of the amino acid composition of the tryptic peptides derived from each of the five peptides and the composition of the four unique methionine-containing peptides from Fc fragment, it is possible to obtain a partial sequence of the Fc fragment. These results suggest that this may be a major sequence for the rabbit Fc fragment.