Summary When the products of digestion of human immunoglobulin G with pepsin are compared to those obtained with papain, one major difference appears to involve the recovery of the Fc fragment. An Fc-like fragment was isolated from peptic digests of a variety of human immunoglobulin G preparations, including 10 different samples obtained from persons with multiple myeloma. Starch gel electrophoresis showed all isolated preparations to be heterogeneous and the patterns unrelated to some known immunoglobulin subgroups and genetic factors. The preparations were Gm(a+) but Gm(y-), Gm(f-) and Gm(z-). They failed to react with rheumatoid factor and serum complement. Their structural and functional characteristics were compared to the Fc and F′c fragments obtained on digestion of immunoglobulin G with papain.