The interaction of cyclosporine A (cyclosporine) with human plasma lipoproteins has been investigated by combining in vitro and in vivo methods. Binding parameters were derived in vitro from an erythrocyte partitioning method, and provided reliable Ka (product of the number of binding sites by the association constant) estimates: high-density lipoprotein, 2.21 +/- 0.48; low-density lipoprotein, 1.23 +/- 0.12; and very low-density lipoprotein, 0.53 +/- 015 liters/g, showing that high-density lipoprotein was the major carrier of plasma cyclosporine. The effects of cyclosporine binding to lipoproteins were investigated in vivo by the intracarotid injection technique of Oldendorf in the rat. The brain extraction of cyclosporine was related inversely to the lipoprotein concentration in the injected solution, allowing estimation of nKa in vivo: high-density lipoprotein, 2.25 +/- 0.59; low-density lipoprotein, 0.62 +/- 0.13; and very low-density lipoprotein, 0.57 +/- 0.14 liters/g. This showed that brain uptake occurred from the free drug pool and possibly from a small part of the originally lipoprotein-bound pool of cyclosporine, at least for low-density lipoprotein-bound cyclosporine. These results allow the calculation of an index of the unbound plasma cyclosporine fraction.