We measured cyclosporine in whole blood samples from renal and heart transplant patients by high performance liquid chromatography and by two radioimmunoassays with use of specified monoclonal antibodies. In particular, we evaluated the analytical performance of a new specific radioimmunoassay with an iodinated tracer. The reproducibility of the method is satisfactory (within-run CV 7.1 to 9.5% and between-run CV 7.2 to 10.3%). The limit of detection is 10.3 micrograms/L and the analytical recovery between 99 and 114%. The results obtained with samples from both renal heart transplant patients agree well with those obtained by HPLC and by a specific RIA that uses a tritiated tracer.