The half–life of plasma cholinesterase (acylcholine acylhydrolase EC 3.1.1.8) was determined in three patients homozygous for the atypical gene for plasma cholinesterase by measuring the rate of disappearance of enzyme activity following intravenous injection of concentrated human cholinesterase. Half–life values of 10.9, 11.1, and 11.3 days were estimated. The distribution volume was estimated to be 18.0, 18.2, and 13.8% of body weight, respectively.