Diphenylcarbamyl chloride, diphenylcarbamyl fluoride, methylphenylcarbamyl chloride, and methylphenylcarbamyl fluoride were studied as acid-transferring inhibitors of chymotrypsin, trypsin, acetylcholinesterase, and serum cholinesterase. The fluorides were the better inhibitors in all cases. They are quite potent inhibitors and are the best of the known inhibitors of chymotrypsin and trypsin. The greater activity of the fluorides is evidence that ther is an electrophilic component in the enzymic mechanism.