Rafaelsen (1) has proposed a membrane transport model involving transport function deficiencies in affective disorders. The deficiencies of transport concern a group pump as well as a specific pump in the cell membrane. As peripheral blood cells are important for the study of basic features essential to biological transport functions and have been suggested as models for membrane mechanisms in the CNS, a series of studies was undertaken to characterize amino acid transport in the human red blood cell (h-RBC). Five different transport systems were identified by kinetic analysis and studies of inhibition and sodium dependence of transport: the L-, T-, Ly-, ASC- and Gly-system. The T-system which appeared specific to L-tryptophan transport has not been identified in other cells. The L-system for large neutral amino acids and the T-system correspond to the group and specific pump, respectively, of the model of Rafaelsen. The importance of the h-RBC as a tool of biological psychiatry is discussed.