The widespread use of organophosphate pesticides creates the possibility of excessive exposure of migrant farm workers to these compounds. Blood cholinesterase determinations were used to compare the organophosphate pesticide exposure of 57 migrant farm workers with that of 35 controls. Frequently reported symptoms of the farm workers which might be related to pesticide exposure were also studied, including headaches, dizziness, loss of weight, nausea, and a general feeling of weakness or loss of energy. Significantly depressed cholinesterase activities were found in the farm workers, with 10.5% of the farm workers having values below the lower limit of normal. There was no significant relationship between frequently reported symptoms of the farm workers and depressed cholinesterase levels.