There is a need for a more effective method for the disinsection of intercontinental aircraft. A study was made of the possible toxic hazard associated with a new method of disinsection using DDVP vapour (O,O-dimethyl-2,2-dichlorovinyl phosphate) as the insecticidal agent. In these experiments, men and monkeys were exposed four times over one- or two-hour periods for a total of 4-8 hours to DDVP vapour in a simulated aircraft cabin. The concentration of DDVP was higher and the exposure periods were longer than those planned for use in disinsection. Concentrations up to 0.7 micrograms per litre of air produced no effect on the cholinesterase of men or monkeys. It was found that a concentration of DDVP of 0.9-3.5 microgram per litre of air caused a slight decrease in plasma cholinesterase of the men and the monkeys. At a DDVP concentration of 7.5-17.9 microgram per litre, monkeys exhibited a marked drop in red cell and plasma cholinesterase and showed miosis, but no other signs of poisoning.