Human plasma cholinesterase (E.C.3.1.1.8) from 1,594 blood donors was phenotyped on the basis of dibucaïne, fluoride, chloride and succinylcholine differential inhibitions according to the criteria of Brown et coll. The observed gene frequencies are: E1u = 0.970,8, E1a = 0.188,0, E1f = 0.103,0.