A group of 31 lettuce harvesters exposed to the organophosphate pesticide mevinphos presented to a local emergency room with moderate cholinergic symptoms and eye and skin irritation, with 22 of the subjects (76%) reporting three or more symptoms. None had baseline cholinesterase values, and plasma cholinesterase activity for all but two workers was above the lower limit of the laboratory normal range. None of the workers received antidotes and all were released for return to work. Twenty-nine workers sought additional care when symptoms persisted, and were followed by the investigators until 12 weeks after exposure. Plasma and red blood cell (RBC) cholinesterase increased until 14 days after exposure. Plasma cholinesterase was estimated to have been inhibited by an average of 15.6% (p less than 0.01), and RBC cholinesterase by 5.6% (p less than 0.01). These findings support the utility of sequential postexposure plasma cholinesterase analyses in confirmation of suspect organophosphate-induced illness when baseline values are not available.