A method for determining erythrocyte cholinesterase titers on small samples of whole blood is described in detail. The method is one which requires little special equipment and can be used by any competent technician. Data for erythrocyte cholinesterase are presented on a group of 101 normal adults, and it is shown that the observed data fit the normal distribution. Eighteen children, aged three months to ten years, all had values within the adult distribution, and the mean value was the same as that for adults. Seven newborn babies all had values at or below the lower limit for adults.