Abstract: An improved colorimetric reagent is disclosed, which is adapted to the determination of the iron content in blood serum. The formulation comprises only two aqueous solutions: one of these comprises a buffer, a soluble salt of an alkali metal or an alkaline earth metal, Chromazurol B, a cationic or nonionic surfactant, whereas the other solution comprises a complexing agent and possibly the same ingredients of the chromogenic reagent for hindering the formation of the disturbing complex between chromazurol B, the surfactant and the iron: the complexing agent is a polycarboxylic acid or a salt thereof.