Endogenous creatinine in serum and urine was measured by adsorption on Lloyd''s reagent and elution with alkaline picrate; the color developed was measured with a Beck-man spectrophotometer (model DU) at 500 m[mu]. This technic eliminates interference from other chromogens and gives 98-100% recovery of added creatinine. Approx. 80% of the material in human serum giving the Jaffe reaction is creatinine; 90-100% of the urinary chromogen is creatinine. The renal clearance of endogenous creatinine detd. by this method agrees closely with the simultaneous inulin clearance, and may therefore be used to estimate glomerular filtration rate.