The apparition of cyclosporine, immunodepressive drug, has largely improved the organ transplantations. However, the range of blood concentrations must be defined to allow the efficacity of cyclosporine therapy and to avoid toxic reactions, because there are very important variations for a same dosage according to the individuals and the diseases. Relative to the low concentrations to be determined (about one hundred ng/ml), the most useful methods for cyclosporine measurement are based on immunochemical assays. This work compare the two methods: radioimmunoassay (RIA) and fluorescence polarization immunoassay (FPIA) simultaneously performed on several hundred samples. A very significant correlation exists between the two techniques (r = 0,80). The advantages of immunofluorescent assay consists in rapidity, sensibility and facility to realize emergency analysis.