Plasma cholinesterase activity was periodically measured in bank blood. Of the initial plasma cholinesterase activity 87% was retained after 21 days of storage at 4 C in citrate-phosphate-dextrose solution. In addition, plasma frozen for 7 weeks at -70 C showed no decrease in enzyme activity. No difference could be demonstrated in the stability of the enzyme related to whether the blood was stored in citrate-phosphate-dextrose or in acid-citrate-dextrose solution. Bank blood transfusion prior to or during succinylcholine administration should not significantly alter the duration of action of succinylcholine, and would probably be effective in the management of prolonged apnea resulting from a deficiency of this enzyme.