A study of the reaction between creatinine and alkaline picrate confirmed that there is a linear relation between absorbance change and creatinine concentration over the entire clinical range of values. The specificity and accuracy of the method are comparable to that of the end-point Jaffé reaction with deproteinization. The test is somewhat imprecise in the low range, but the simple, rapid procedure is appropriate for the monitoring of patients with glomerular disease and those undergoing therapeutic dialysis.