Fifteen patients demonstrating unexpected prolonged apnoea lasting several hours after succinylcholine have been treated by a new preparation of human serum cholinesterase. Adequate spontaneous respiration was re-established in an average period of ten minutes after the injection. In 12 patients biochemical genetic examinations confirmed the presence of an atypical serum cholinesterase. In three patients none of the usual variants were found. It is therefore supposed that other unknown variants of serum cholinesterase exist which cannot hydrolyze succinylcholine. The use of serum cholinesterase in succinylcholine apnoea provided considerable relief to both patient and anaesthetist.