The transport specificity of system y+L of human erythrocytes was investigated and the carrier was found to accept a wide range of amino acids as substrates. Relative rates of entry for various amino acids were estimated from their trans-effects on the unidirectional efflux of L-[14C]-lysine. Some neutral amino acids, L-lysine and L-glutamic acid induced marked trans-acceleration of labeled lysine efflux; saturating concentrations of external L-leucine and L-lysine increased the rate by 5.3 +/- 0.63 and 6.2 +/- 0.54, respectively. The rate of translocation of the carrier-substrate complex is less dependent on the structure of the amino acid than binding. Translocation is slower for the bulkier analogues (L-tryptophan, L-phenylalanine); smaller amino acids, although weakly bound, are rapidly transported (L-alanine, L-serine). Half-saturation constants (+/- SEM) calculated from this effect (L-lysine, 10.32 +/- 0.49 microM and L-leucine, 11.50 +/- 0.50 microM) agreed with those previously measured in cis-inhibition experiments. The degree of trans-acceleration caused by neutral amino acids did not differ significantly in Na+, Li+ or K+ medium, whereas the affinity for neutral amino acids was dramatically decreased if Na+ or Li+ were replaced by K+. The observation that specificity is principally expressed in substrate binding indicates that the carrier reorientation step is largely independent of the forces of interaction between the carrier and the transport site.