The contribution of four transport pathways to Li+ influx in human, chicken and eel erythrocytes was studied. All pathways were measured simultaneously in each kind of blood sample to avoid possible temporal variations in transport parameters. We found that: 1) Li+ influx via Na(+)-K+ pump increased 2-3 fold in the order human < eel < chicken; 2) by the countertransport exchange system lithium influx increases its variance between 15% and 35% when saline medium is replaced by choline Cl; 3) the anion exchange system (band 3 protein) shows very little interindividual variability on the lithium influx; 4) the lithium leak pathway, for human and eel red blood cells, is the major contribution, whereas it is negligible for chicken erythrocytes. It is concluded that a similar transport system exists in the red cell membranes of the three species which can transport lithium. However, the exchange system does not exhibit identical transport characteristics in the three species and shows a marked inter- and intra-species variability in maximum transport capacity and some differences in susceptibility towards the inhibitors.