According to the development of monoclonal antibodies against cyclosporine, it became avaiiable to replace the conventional polyclonal antibody method with new monoclonal antibody method to measure the blood leve1 of cyclosporine by radioimrnunoassay. We compared the results obtained hy the two methods: polyclonal antibody and monoclonal antibody method. The results were ohtaincd as follows: 1) We obtained mean value 137,3 ng/ml and CV 16.1% from plasma sample, and mean values 495.7ng/ml and 1053.8 ng/ml and CVs 19.3% and 17.4% respectively from whole blood sample by polyclonal antibody method. 2) For the two control groups, 100 ng/ml 400 ngjml each, we obtained that the CVs were 20,2% and 14.0% respectively from plasma sample, and 11.9% and 13.1% respectively from whole blood samaple by monoclonal antibody method. In conclusion, we found that ryclosporine RIA was a relatively reliable method to measure blood or plasma concentration. Especially RIA using monocloanl antibody showed less degree of error in measurement compared to polyclonal antibody method.