PURPOSE: To stabilize the subject compound useful as a clinical examination reagent, etc., such as determination of guanase in high stabilization efficiency by adding a salt or a chelate compound of a metal such as iron, copper, cobalt and manganese to a solution containing xanthine oxidase.

CONSTITUTION: Xanthine oxidase useful as a reagent for the determination of guanase, adenosine deaminase, superoxide dismutase, inorganic phosphorus, etc., can be stabilized by adding a salt (including complex salt) of a metal such as iron, copper, cobalt and manganese and/or a chelate compound of the above metal to a solution containing xanthine oxidase. The compound to be used as the stabilizing agent is e.g. ferrous chloride, ferric chloride, cupric chloride, cobalt chloride, manganese chloride, potassium ferricyanide, Mn acetylacetonate, cobalt acetylacetonate or a chelate compound of the metal with a chelating agent such as ethylenediaminetetraacetic acid.