The efficacy of various Substrates for measuring serum choline esterase for the evaluation of hepatic function was studied using 0-toluoyl-and succinyl-choline, and acetyl, butyryl-and propionylthiocholine. In hepatic disease, the serum choline esterase activity with these Substrates was decreased at a similar rate, showing no significant difference. In 78 -84% of cases with hepatic cirrhosis the enzyme activity with these Substrates was less than 50% of the average level of normal individuals, but in acute and chronic hepatitis only 4-9 and 12-14% of patients showed these lower values, respectively. The present study indicates the usefulness of sequential monitoring of serum choline esterase activity with any of these Substrates for assessing hepatic disease, particularly cirrhosis, and for monitoring the course of hepatic disease.