The activities of plasma esterase and phenotypes were studied in a family of four in which the mother had previously responded to the neuromuscular blocking agent succinylcholine with a prolonged paralysis. The plasma of the mothei contained atypical esterase which hydrolyzed succinylcholine and other esters at reduced rates. The increased response to succinylcholine thus relates to an atypical cholinesterase and a lowered total esterase activity. Three phenotypes were indicated by data obtained from dibucaine and fluoride inhibition studies, but each of the four family members had different substrate activity patterns for plasma esterase, as determined by K m values. The study reported here presents an example of two siblings with the same phenotype but with markedly different plasma esterase activity toward a variety of choline and noncholine esters. The father and the one son who have the least difference in enzyme activity patterns have distinctly different phenotypes, as determined by fluoride and dibucaine inhibition. The study emphasizes the need for additional characterizations of atypical esterase within families so that inheritance in relation to plasma esterase may be further understood.