A new fluorimetric method for determining creatine in serum, blood, and urine is described. The method is based upon the coupling of guanidine compounds with ninhydrin in alkaline solution to produce highly fluorescent products. In degree of sensitivity and simplicity the method possesses several advantages over currently used colorimetric methods, and creatine can be determined in concentrations as low as 1.0 X 10-7M. The serum creatine concentration was found to be 0.41 mg.% in normal adult males, who excrete only small amounts of creatine in the urine.