SummaryMethods and conditions suitable for determining human serum aldolase have been studied. It is concluded that the method of Sibley and Lehninger, while not suitable without some modification for making comparative studies of aldolase of various tissues, is nevertheless well suited to the determination of serum aldolase, especially if the pH is altered to 7.2. Serum aldolase must be determined at a temperature of 38°C or higher in order to obtain a sufficient reaction rate for successful measurement, although this temperature is too high for the determination of crystalline aldolase or the aldolase of muscle homogenate.