An episode of acute insecticide poisoning in California agricultural workers was described. Thirty one of 44 farm workers and three agricultural officials developed dizziness, visual disturbances, headache, nausea, and eye irritation after entering a field of iceberg lettuce which had been sprayed earlier that day with mevinphos (7786347). Examination and plasma cholinesterase (ChE) testing at a local hospital resulted in the hospitalization of two workers because of breathing difficulties. Two other workers had plasma ChE values below the normal level. The remaining workers disrobed, hosed down with water, and were released; none were advised against returning to work the next day. Because several workers were unable to return to work the next day because of continuing symptoms, 29 workers were examined at a second hospital and tested for plasma ChE activity. One worker with bradycardia was hospitalized. Symptoms such as eye irritation, headache, visual disturbances, dizziness, nausea, fatigue, chest pain or shortness of breath had prevalences of at least 21%. Twenty two workers reported experiencing at least three of these symptoms. Plasma ChE and erythrocyte acetylcholinesterase (AChE) were initially depressed, but no values were below the lower limit of the normal range. ChE and AChE values increased significantly over the next 2 weeks. A retrospective evaluation based on the followup measurements indicated that during the acute stage of the incident plasma ChE and erythrocyte AChE had been decreased by a mean of 15.6 and 5.6%, respectively. The authors conclude that the observed decreases in ChE and AChE activity in the workers were probably caused by mevinphos exposure.