Cyclosporin A treatment is unimaginable without continuous and periodical monitoring of the blood cyclosporin concentration. One of the best methods for measuring is the radioimmunoassay, because this method makes possible the monitoring of a large number of samples with good sensitivity. The method is suitable for measuring either only the cyclosporin A concentration (specific monoclonal antibody) or the cyclosporin A and its metabolites (nonspecific monoclonal antibody). The use of 125I labelled tracer has several advantages such as increased sensitivity, fast sample counting and so fast measuring of the concentrations.