A kinetic method for the determination of creatinine in 50 ul of serum or plasma, without deproteinization or pre-treatment, at rate of 50 to 100 samples an hour is described. The precision is better than2%(CV) in the normal range and the recovery very close to 100%. The method is linear at least up to 10−3M(11.3 mg/lOOml) and can be standardized with aqueous creatinine solutions. Glucose and proteins do not interfere with the reaction.