We show how iron and copper ions may be simultaneously measured in 0.2 ml of serum after using guanidine hydrochloride to cause their release from protein. Sensitive reagents for iron (2,4,6-tripyridyl-s-triazine) and copper (disodium 2,9-dimethyl-4,7-diphenyl-1, 10-phenanthroline disulfonate) facilitate such measurement. We eliminated spectral interference between the two metal-ion complexes by selective use of pH. Results compare well with those obtained by accepted methods in which the concentrations of these ions are measured independently.