In 1951, 258 persons with different degrees of agricultural exposure to organic phosphorus insecticides were studied in regard to blood cholinesterase values and symptomatology. The average cholinesterase values for groups known to have definite and consistent exposure showed significant reduction during the period of exposure. Average erythrocyte enzyme values for mixing-plant personnel, commercial applicators, and part-time applicators were 0.46, 0.65, and 0.69, respectively, as compared with 0.72, the lowest average value found for any group during the preexposure period. The average cholinesterase values for groups with little or no exposure to insecticides remained normal or showed changes so small that their significance is in doubt. Although extensive day-to-day exposure was regularly associated with reduced blood cholinesterase, the only fatal and near-fatal cases which were found were associated with brief, massive exposure and gross carelessness. Illness characterized by myosis or by any three of a group of selected symptoms was considered suggestive of poisoning but not absolutely diagnostic of it in isolated cases. Such illness was increased in the definitely exposed groups during the period of their exposure.