1. Peptide maps of Fc fragments or heavy chains of 36 G myeloma proteins and two "heavy chain disease" proteins belonging to the four γ-chain subgroups revealed very striking similarities between them. However differences in a few peptides were noted. This was most pronounced for the Ge(γ2d) subgroup which lacked three peptides characteristic of the other three subgroups. While Fc fragments from different proteins belonging to the same subgroup appeared very similar, minor differences in addition to those based on currently recognized Gm factors were occasionally noted.
2. Fc fragments from Gm(a+) We(γ2b) proteins had a peptide previously shown to be characteristic of normal Gm(a+) γG-globulins. Fc fragments from Gm(a-) molecules belonging to the We(γ2b), Vi(γ2c), or Ne(γ2a) subgroups, whether Gm(b+), Gm(f+), or Gm(-), had the peptide previously identified in Gm(b+f+) normal γG-globulin. This "non-a" peptide was absent in peptide maps from Gm(-) molecules of the Ge(γ2d) subgroup which contained instead another peptide with the same electrophoretic mobility but migrating slightly further on chromatography.
3. Both the "a" and "non-a" peptides were pentapeptides having three amino acids in common, and differing in the other two. The "a" peptide contained one residue of lysine, aspartic acid, threonine, leucine, and glutamic acid. The "non-a" peptides prepared from Gm(b+), Gm(f+), and Gm(-) proteins were identical and contained one residue of lysine, threonine, and methionine sulfone, and two residues of glutamic acid.
4. Several possible mechanisms for the origin of these differences, and their possible role in serologic specificity are discussed.