A highly sensitive monoreagent for the determination of iron and total iron-binding capacity (TIBC) in serum is described in this paper. It contains the dye chromazurol B which gives with iron a high molar absorption complex (A630nm = 1.6 X 10(5) l/mol cm). The more complex problem in the study of the reagent was to make negligible the interferences of the substances in the sera in order to get the procedure without blank samples or deproteinization. Until now none of the reagents presents these peculiarities, that might render easier both the manual analysis and the application to the automatic apparatus. Furthermore, the stability of the reagent (2 years) and the reliability of the results, as detected by data on accuracy, precision (coefficient of variation = 1.9%), linearity (up to 110 mumol/l) and correlation with three different methods, indicate that this procedure is particularly suitable for routine clinical chemistry.