Abstract: A colorimetric reagent formulation is disclosed for the determination of the ferric iron content of serum. The formulation consists of two aqueous solutions. One solution, acting as chromogenic reagent, consists of a buffer, a soluble salt of an alkali or alkaline earth metal, chromazurol S in acidic form, and either a cationic or nonionic surfactant. The other solution, acting as masking reagent, consists of a complexing agent and if desired the same components of the chromogenic reagent, which are able to prevent the formation of the complex between the chromazurol S, the surfactant and the iron. The complexing agent consists of a polycarboxylic organic acid or salt thereof.