A method for the analysis of blood and urine creatinine, on the Beckman DSA-560 analyzer, is described. It is based upon the measurement of the rate of formation of Jaffe's coloured compound, between 60 and 210 s after the addition of picric acid to samples diluted with an alkaline buffer. The method requires no deproteinization; the reagents are stable and analyses can be made at a rate of 120 samples/h using 100 μl of plasma.