Analysis of urea by urease was investigated and an improved method available to estimate the amounts of 1 to 10 γ of urea was established. 1) Colorimetry of ammonia by indophenol method was adaptable to acidic solution obtained by diffusion analysis. 2) Considerable amounts of ammonia and ammonia-like nitrogen were found in some commercial albumins in which urease was dissolved to be activated. 3) It was found that urease was activated by β-mercaptoethanol as much as by albumin. 4) It was scrutinized to estimate urea which was contained in plant.