Serum and red cell cholinesterase activities were determined in 2 groups of subjects namely:-I) a group of 10 patients who took organophosphate insecticide and were admitted into the hospital for treatment and II) a group of 65 workers from an organophosphate insecticide factory. Serum cholinesterase levels were considerably depressed in all patients in group I and one patient died. The low serum cholinesterase activities increased very slowly and were still very low on day 4 of admission. Serum cholinesterase level in 65 workers were significantly lower than that of the normal subjects. The exposed subjects had still further lower serum cholinesterase activity than those of the non-exposed subjects. There was no significant difference between their red cell cholinesterase activities and those of the normal subjects. Serum and red cell cholinesterase levels in these workers also showed no correlation to the duration of insecticide exposure. These findings indicated that serum cholinesterase activity was a good diagnostic aid in acute exposure because it responded more rapidly than red cell cholinesterase level but it was not sensitive for follow up of the treatment since its recovery rate was too slow. Findings of low serum cholinesterase with normal red cell cholinesterase levels without signs or symptoms of toxicity indicated that these workers had been exposed to some degree of organophosphate insecticides.