Summary A method for determination of ceruloplasmin in serum is presented in which rate of oxidation of p-phenylenediamine is measured spectrophotometrically or photometrically. Correction for catalytic oxidation by free metals present is made by reading the test against a serum blank in which enzymatic activity is inhibited by azide. The effects of pH and temperature on the reaction have been investigated and normal values obtained by our method. Ethylenediaminetetraacetate significantly inhibits enzymatic oxidase activity of serum.