Amino acid sequence analysis of the pFc′ fragment obtained by pepsin digestion of a human IgG2 myeloma protein PIG Gm (n or 23) negative shows it to consist of 112 residues. It starts at position 334 (γ1 numbering) and contains eight residues from the Cγ2 region, and the whole Cγ3 domain. Comparison with the sequence of γ1 shows two differences at positions 339 and 397. Each of them can be explained by a single base substitution. This high degree of homology among γ-chain subclasses suggests a recent diversification.