The inhibition of cholinesterase (ChE) activity by the organic phosphate insecticides provides a sensitive indicator of absorption of these compounds. In order to interpret the laboratory results of the ChE activity, one must consider several factors, including the accuracy of the determination. Unless the laboratory personnel are accustomed to performing the analysis (Michel method), a duplicate sample should be sent to a laboratory where the test is commonly done for comparison. Blood is collected in a heparinized capillary tube, and the plasma and red blood cells (RBC) are separated as soon as possible. The test may be performed with as little as 0.02 ml of heparinized blood.1 Since there is a 100% variation in the normal ChE activity among different individuals, and since the same person may show a variation of as much as 25% at different times, it is extremely helpful if the normal values for the individual