Nineteen subjects were investigated as a result of apnoea following the administration of suxamethonium occurring in one member of the family. Examination of the blood for cholinesterase activity, dibucaine number, and fluoride number was carried out and the appropriate genotype determined. The significance of the genetic variation of plasma cholinesterase in relation to suxamethonium apnoea is discussed. Five subject had the genotype Eu1Eu1, seven were of genotype Eu1E1a, three were Eu1Es1, and four Ea1Es1. Nine members of the family were considered to be at risk for the administration of suxamethonium. The appropriate action to safeguard other members of the family following a case of apnoea is outlined. By routine preoperative blood examination, 92 of 100 cases of suxamethonium apnoea resulting from a genetic abnormality of plasma cholinesterase can be anticipated.