Abstract A method is described for the determination of creatine in blood serum with creatine phosphokinase on the principles of that of Tanzer and Gilvarg 1 . Serum is treated with a solution of iodine and then protein is removed with barium hydroxide and zinc sulfate. Evidence is presented to show that pyruvate of serum does not affect the results under the conditions of the method. The results of 20 recovery experiments show that 86 to 110% of added creatine can be recovered. Values of creatine determined in 13 normal women and 6 normal men range from 4.53 to 10.2 μg/ml and average 6.70 μg/ml. The possibility of determining creatine with creatine phosphokinase in ultrafiltrates prepared from serum treated with iodine is also discussed.