In order to evaluate the health hazard due to exposure to organophosphorus insecticides, we measured the blood cholinesterase activity ana urinary para-nitrophenol among 56 exposed subjects. They are orchard workers, rice plant workers and smithion factory workers. The clinical symptoms were also checked by physicians. We also measured the blood cholinesterase activity and urinary para-nitrophenol excretion of 20 urban people and 15 rural people who had never been exposed to organophosphorus insecticides in order to compare them according to age, sex and geographical differences. And these results were also compared with those of exposed groups. The results obtained were as follows. 1. The normal plasma cholinesterase activity and cell cholinestrase activity were $0.861{pm}0.148;{Delta}pH/hr$ and $0.822{pm}0.154;{Delta}pH/hr$. And normal para-nitrophenol in urine was $1.21{pm}0.52mg/liter$. 2. No significant difference was existed in blood cholinesterase activities and urinary para-nitrophenol excre tion according to sex, age and geographical difference. 3. The plasma cholinesterase activity and cell cholinesterase activity of orchard workers, rice plant workers and smithion factory workers were $0.682{pm}0.189;{Delta}pH/hr,;0.775{pm}0.160;{Delta}pH/hr,;0.754{pm}0.123;{Delta}pH/hr,;and;0.691{pm}0.082;{Delta}pH/hr,;0.756{pm}0.117;{Delta}pH/hr,;0.739{pm}0.117;{Delta}pH/hr$. And significant decreses in blood cholinesterase activities were existed among orchard workers and smithion factory workers compared with control group. 4, The urinary para-nitrophenol excretions of orchard workers, rice plant workers and smithion factory workers were $1.33{pm}0.66mg/liter,;1.19{pm}0.88mg/liter;and;1.37{pm}0.67mg/liter$ and there were no significant difference between exposed groups and control group. 5. The clinical symptoms complained during and after organophosphorus insecticides exposure were frequently ranked by headache (67.7%) and vertigo (64.5%) and muscular ataxia and weakness (51.6%).