Determination of serum creatinine by the Jaffé reaction may give erroneous results because of interfering noncreatinine chromogens. Studies of the Jaffé reaction in alkaline picrate media of various pH demonstrated that creatinine, proteins, and certain chromogens have characteristic properties as regards the appearance and intensity of the color reaction. Thus a method for determining "true" creatinine has been developed in which simple spectrophotometric measurements are made at 500 nm in two alkaline picrate reagents, buffered at pH 9.65 and 11.50. Results agree well with those obtained by the use of an ion-exchange method. The proposed method is easy and rapid and offers the advantages of increased specificity and direct analysis of serum and urine.