Total chromogen, true, and AutoAnalyzer methods of measuring serum and urine creatinine by the Jaffe reaction were investigated. Some factors influencing this reaction were examined. These included wavelength, blank, linearity, and conditions of color development. Modifications of the three methods were made and their precision, recovery, and sample stability determined. The interference of ketones and glucose were measured. Finally, the values obtained by the three methods on the same samples of serum and urine were compared statistically.