The therapeutic use of cyclosporine (CsA) requires total blood levels to be monitored with a specific assay, since this drug is nephrotoxic, has a narrow therapeutic index, shows wide intra- and inter-individual differences in its pharmacokinetic behaviour and can interact with other drugs. The parameter generally measured is the residual blood concentration at equilibrium; levels are occasionally again measured four or six hours after CsA administration. Patients are monitored every two days during the first two weeks following organ transplantation, a frequency which gradually declines to monthly determinations at four to six months in the absence of complications. The therapeutic range of blood CsA concentrations is still poorly defined. In renal, liver, heart and bone marrow transplantation most authors recommend concentrations of 100-250 ng/ml for the first three months when renal function is satisfactory. Ways of administration (i.e. continuous infusion or dividing up of the daily dose) must be taken into consideration to assess values of blood concentration of CsA.