More than 70% of cyclosporine A (CsA) is bound to erythrocytes at whole blood concentrations of 50-1000 ng.ml-1. Cytosolic CsA is bound to the erythrocyte peptidyl-prolyl cis-trans isomerase cyclophilin. Measurements of serum CsA levels under clinical conditions are hampered by a temperature-dependent translocation of CsA into erythrocytes during cooling of the probes to room temperature. In order to characterize the kinetics of CsA uptake and to find a specific uptake inhibitor, we developed a method to measure the velocity of uptake based on rapid cooling of the erythrocyte suspension. The total erythrocyte-binding capacity for CsA amounted to 43 x 10(-5) nmol per 10(6) erythrocytes or 2.6 x 10(5) molecules per erythrocyte. Whereas the erythrocyte-binding capacity of CsA was temperature-independent between 10 degrees C and 42 degrees C, uptake kinetics of CsA were temperature-dependent. The Arrhenius plot for CsA uptake in human erythrocytes was linear and no transition temperature between 0 degree C and 42 degrees C could be detected. Therefore the CsA uptake process in human erythrocytes did not fulfil the criteria of carrier-mediated transport. This indicates that CsA diffuses passively into human erythrocytes. Hence, erythrocyte CsA uptake cannot be specifically inhibited.