The importance of control materials in intralaboratory quality assessment of cyclosporine A determination was analyzed. Two types of control materials and a human pool serum were evaluated. The first material was a two-level lyophilized cyclosporine human whole blood control (TLCC); the other was a tri-level therapeutic drug monitoring control (TDMC). The drug was determined in all specimens by both tritiated and iodinated radioimmunoassay. The coefficients of variation obtained in TLCC samples were significantly lower in comparison with the TDMC samples and similar to that of the human pool.