An analysis of investigations performed between December 1978 and September 1982 into the cholinesterase status of 795 Caucasian patients has revealed an E1aE1j genotype in three (0.4%) and an E1aE1k genotype in 22 (2.8%). Both groups of patients are at increased risk of sensitivity to suxamethonium. Inhibitor numbers characteristic of these genotypes are reported which it is hoped will assist other workers to identify them more easily. While the J allele is probably rare among the general population it is suggested that as many as one person in 76 could be a KK homozygote. Our findings provide a possible explanation of the low cholinesterase activities seen in some patients for which there is no other obvious cause.