Cyclosporin A (CyA) undergoes saturable binding to erythrocytes. Theoretically, measurement of erythrocyte concentration can be used to estimate the unbound concentration, which in turn may be more closely related to response than total plasma concentration. This possibility for estimating unbound concentration was explored in 139 blood samples taken from renal transplant patients who received CyA therapy. In practice, the method proved to be no better than that obtained by assuming a constant fraction CyA in plasma unbound. In this study, however, both methods were too imprecise to be of practical value. If unbound CyA concentration is to be determined it must be measured directly or must be estimated by multiplying total plasma concentration by fraction of drug unbound.