1. Comparison of peptide maps of the Fc fragments of normal G immunoglobulins and 11 G myeloma proteins of the We (b) type showed them to be very similar except for differences associated with the Gm type. Some additional differences were noted, however, in the Fc fragments of three Vi (c) myeloma proteins.
2. Peptide maps of heavy chains from the same G myeloma proteins differed from each other and from normal heavy chains. In general, the myeloma chains contained a larger number of well defined spots; some of these were common to normal heavy chains while others were unique to each protein. Others, present in normal heavy chains, were lacking in the myeloma proteins.
3. Comparison of the heavy chains and Fc fragments from the same protein suggests that much of the variability of different myeloma proteins and, presumably, antibodies resides in the Fd fragment.
4. Further support for this is given by the finding that the antigenic specificity of 3 myeloma proteins also appeared to reside in the Fd fragments.