The availability of the immunosuppressant cyclosporin has led to significant improvements in the recent success of clinical organ transplantation. Problems associated with cyclosporine therapy include serious adverse reactions, such as nephrotoxicity, wide variability in the drug's pharmacokinetics, and several complex drug interactions. Monitoring of drug concentrations is accepted as a part of the routine care of patients receiving cyclosporine. However, cyclosporine concentrations can be determined in different biologic fluids by either radioimmunoassay or high-performance liquid chromatographic techniques. Controversy exists regarding the optimal analytic technique to be used for cyclosporine monitoring and pharmacokinetic studies. This commentary addresses factors including: why the monitoring of cyclosporine concentrations is important, the differences between the biologic fluids and analytic techniques, when monitoring and special pharmacokinetic studies are indicated, what some major transplant centers have established as a "therapeutic range" for cyclosporine concentrations, and provides guidelines for the optimal clinical monitoring of cyclosporine concentrations.