Some progress has also been made in identifying polypeptides of the human red cell membrane that may be responsible for specific membrane functions. Although the data are just becoming available it is possible to provisionally identify polypep tides that may be responsible for lipid bilayer stability, receptor function, and possibly even transport across the membrane. Detailed structural studies on some of these proteins are under way and the progress to date is summarized here. It seems safe to predict that investigators of the red cell membrane will soon be able to carry out serious structure-function studies in several areas. A provisional structure for one of the major receptor molecules of this membrane is already in hand, and others will surely follow. Detailed'chemical studies of macromolecules thought to be transport components are now under way in several laboratories. Mutant forms of red cells have not played any role in the studies to date, but there are a variety of genetic red cell abnormalities in humans that could be studied with profit. Red cells of patients with hereditary spherocytosis may have defects in the structural proteins that normally stabilize the lipid bilayer, and some understanding of their changes could lead to ideas as to how the normal forms of the same molecules provide support to the membrane.