Abstract
1. Factors influencing the isocitric dehydrogenase (ICD) activity of serum were studied. Substrate inhibition by high concentrations of isocitrate was observed. 2. The temperature-activity curve of serum ICD was determined. The importance of controlling the temperature of the reaction was stressed. 3. Using this information, a method of measuring the ICD activity in 0.2 ml. of serum is described. 4. The range of ICD activity in 50 healthy males was 30 to 192 units with a mean of 90 units and a standard deviation of ± 34.3 units.