Widening agricultural requirements for anticholinesterase insecticides that represent a potential health hazard, and parallel diagnostic problems that will arise if the new anticholinesterase war gases are used have created the need for an objective clinical test applicable in field surveys and in differential diagnosis for individual patients. The symptoms of anticholinesterase intoxication, whether resulting from exposure to insecticides or nerve gases,1 may appear too late to provide an adequate warning, or may be too nonspecific to serve in all instances as an accurate and timely guide to definitive diagnosis. For the benefit of commercial spray operators and of others possibly exposed to anticholinesterase chemicals, a micro-blood-sampling method was used in the summer of 1951 by several United States and Canadian health agencies. The procedure minimized the labor of screening studies, because it required no syringes and needles for venipuncture and permitted shipment of capillary rather than test tube size