A simplified method for the in vitro determination of TIBC and UIBC in serum using 59Fe was subjected to quality control in the laboratory and clinically studied in duplicate determinations of 185 sera of an unselected patient population. The data obtained were investigated by statistical means (regression, correlation, variance analysis). The precision of the method was evaluated step by step. The results indicate that the radiochemical determination of TIBC values is equivalent to the "indirect" one (UIBC + SI) as far as accuracy is concerned. Advantages of the radiochemical method are discussed.