Biological monitoring and the control of safety precautions against organophosphorus compounds (parathion, malathion, and methylparathion) was performed through the measurement of S-cholinesterase (E.C.3.1.1.8) in personnel removing poisonous depots. Depressions in S-cholinesterase activity where found in a majority of 62 workers in comparison to their preexposure values. After the last exposure the S-cholinesterase activity of the workers returned to the individual preexposure values; a few individuals needed about half a year for their S-cholinesterase activity to reach the preexposure level. As no workers showed either pathological values for the enzyme or poisoning symptoms, it was concluded that the measurement of S-cholinesterase activity is an adequate and sensitive method for monitoring and controlling exposure to organophosphorus compounds.