PURPOSE: To perform the colorimetric quantitative analysis of ammonia using a color forming component and an oxidizing agent stably and economically with high accuracy, by using peroxidase or catalase as a reaction promoter.

CONSTITUTION: In the quantitative analysis of ammonia using a color forming component and an oxidizing agent, peroxidase or catalase is used as a reaction promoter. This method is different from a conventional one using sodium nitroprusside and, even if said promoter is used in the same solution along with urease, a phenolic compound and an N,N-disubstituted anilin compound, not only urease and peroxidase but also other coexisting compounds receive no adverse effect to each other at all and a measuring reagent becomes extremely stable and, therefore, not only urease but also peroxidase or catalase may be used only in a necessary amount.