1. The initial rate of L-tryptophan uptake into human red cells as a function of the concentration in the medium was studied at 25 and 37 degrees C. 2. Uptake was resolved into saturable and linear components. Kinetic constants at 37 degrees C were, apparent Km 1.55 mM, V 0.145 mmol/l cell water per min and apparent KD 0.0103 min-1. 3. Inhibitor studies showed that L-tryptophan transport via the saturable component represents uptake by a previously unidentified transport system, designated the T-system. The linear component represents L-tryptophan transport via the L-system. 4. The substrate specificity of the T-system is apparently limited to the aromatic amino acids, L- and D-tryptophan, L-tyrosine and L-phenylalanine. The main route of L-phenylalanine transport is, however, via the L system. L-Tyrosine is partly transported via the T-system, partly via the L-system.