A kinetic method is reported for the determination of creatinine in serum without deproteinization, using alkaline 3,5-dinitrobenzoate solution. The increase in absorbance at 546 nm due to the formation of an orange-red complex is measured. The relationship between the creatinine concentration and absorbance was linear up to a tested concentration of 8840 mumol/l in the aqueous standard solution. Recovery of added creatinine was 98-102%. Compared with the procedure of Jaffé, as modified by Helger et al. (Z. Klin. Chem. Klin. Biochem. 12, 344-349 (1974], the present method shows less interference by pseudo-creatinine chromogens, and no interference by high serum concentrations of bilirubin or triacylglycerols. The measured serum concentration of creatinine was not affected by cephalosporin therapy. Interferences and side reactions have been minimized or eliminated by optimization of the concentrations of all reactants and of the measurement time. The described procedure can, however, only be performed in an instrument, which permits the measurement of absorbance changes in short time intervals after the start of the reaction. In comparative studies on 212 sera, the dinitrobenzoate method and the enzymic UV-test gave similar results, whereas the Jaffé method as modified by Helger et al. gave significantly higher values.