A radioimmunoassay for the immunosuppressant drug Cyclosporin A has been developed which makes possible the monitoring of the drug by direct measurements in clinical plasma and serum samples. The antisera have been produced in rabbits using the hemisuccinate derivative of a structural analogue of Cyclosporin A as a hapten. The assay has both adequate specificity and sensitivity for Cyclosporin A to be suitable for the routine monitoring of therapy. Some degree of cross-reactivity has been shown to occur with four metabolites which were isolated from urine samples.