A simple, rapid, and sensitive method for the determination of iron in serum or plasma is described.The procedure is carried out at room temperature with 2 ml. of serum or plasma, or with 1 ml. if high values are expected; it can be applied to turbid or jaundiced samples, whether previously frozen or not.An ethanolic solution of 4: 7-diphenyl-1: 10-phenanthroline is used to produce a coloured iron complex, the optical density of which can be measured in any suitable photometer, using either 10 or 20 mm. fused glass cuvettes or matched tubes of 1.1 cm. internal diameter.