Plasma and red cell cholinesterase activities were determined in 250 male and 100 female healthy adults by a continuous pH-stat automatic titration method. A study of plasma and red cell chollnesterases was then carried out in sick adults and children and in healthy pregnant women admitted to hospital wards disinsected with “Vapona Strips.” A moderate decrease in plasma cholinesterases was observed only in subjects exposed for 24 hours per day to DDVP airborne concentrations above 0.1 mg/cu m and in patients with liver insufficiency. In all subjects examined, even in those whose plasma cholinesterase dropped, the red cell cholinesterase was unaffected. The use of garments kept in cupboards disinsected with Vapona Strips did not produce changes in either plasma or red cell cholinesterases. © 1969 Taylor & Francis Group, LLC.