A differential decomplexation technic for the determination of serum zinc in filtrates was studied. It involved the masking of zinc, copper, and iron by the addition of cyanide to a buffered aliquot of an acid filtrate, followed first by the destruction of the excess cyanide and then the cyanide attached to zinc. The release of ionic zinc allowed reaction with the color reagent to take place. Quantitation with internal blanking was thereby accomplished before interference by other cations became possible.