Sequential postexposure cholinesterase (CE) analyses were evaluated for the confirmation of organophosphate poisoning among field workers. Thirty-one male Hispanic workers began harvesting iceberg lettuce 2 hours after the application of mevinphos (7786347) (MP). Two hours later, the workers began to experience eye irritation, headache, visual disturbances, dizziness, nausea, vomiting, weakness, chest pain or shortness of breath, skin irritation, pruritis, eyelid fasciculation, arm fasciculation, excessive sweating, and diarrhea. Episodes of headache, dizziness, visual disturbances, nausea, vomiting, and other symptoms were reported through the study that lasted 12 weeks. Plasma CE activity increased by an average of 0.051 Michel units the first week and by afterwards. The plasma CE was inhibited by an estimated average of 15.6 percent. Red blood cell CE increased by an average of 0.046 Michel units within 14 postexposure days. The plasma activity regenerated at a rate of 1.20 percent per day, while the red blood cell CE activity increased by 0.43 percent per day. Interindividual and intraindividual variations were a much greater source of variation than the batch effect in CE analysis. The persistence of complaints was conspicuous. Since organophosphate and carbamates have a cumulative effect, CE inhibited persons are at an increased risk upon repeated exposure.