1. The effect of histidine and cysteine on the initial rate of zinc influx into rat and human erythrocytes in the presence of bovine serum albumin has been investigated. 2. The L-enantiomers of both amino acids promoted zinc influx into rat erythrocytes in a dose-dependent manner. L-Histidine, but not L-cysteine, also promoted zinc uptake into human erythrocytes. D-Histidine did not promote zinc uptake in either rat or human erythrocytes. In rat erythrocytes D-cysteine was significantly less effective than L-cysteine. 3. The stimulation of zinc influx into rat erythrocytes by 20 mM L-histidine was approximately 4.1 times that seen with human erythrocytes. 4. The influx of zinc in the presence of varying concentrations of L-histidine was linearly related to the calculated concentration of the zinc-bis-histidine complex but not to that of the zinc-mono-histidine complex or the free ionic zinc concentration. 5. These results are discussed in relation to the nature of the transport mechanisms involved.