The distribution of cyclosporin A between plasma, leucocytes and erythrocytes was studied in vitro by means of sedimentation in Ficoll-Paque and dextran. The uptake by erythrocytes was found to be about 50% and the fraction of cyclosporin A bound to leucocytes amounted to 15%. Fractionation of plasma by ultracentrifugation also showed that two thirds of the drug were associated with lipoproteins whereas binding studies with isolated lipoproteins and plasma also indicated that lipoproteins were the major complexing constituents for cyclosporin A in plasma. The binding of cyclosporin A to erythrocytes and lipoproteins seems to be a linear process. The binding to the leucocytes may be a saturable process, however it is of minor importance in terms of the overall binding capacity in the blood.