The Gm groups, first delineated 5 years ago, are attracting increasing attention from workers in diverse disciplines. Geneticists, anthropologists, medico-legal authorities, immunologists, rheumatologists, protein chemists and physical chemists are now actively working in this field. In addition, hematologists may find the possibility of Gm incompatibility as a cause of transfusion reactions worthy of investigation. It is probably that more Gm groups will be discovered, and that the system may eventually rival the blood groups in complexity. The physico-chemical nature of the differences in the human 7S gamma globulins and the evolutionary basis for the selection of the phenotypes have yet to be elucidated. Also, it would seem important to investigate the role of ecological and geographical factors on the distribution of the Gm alleles in various ethnic groups.