Increased attention in recent years to poisonings caused by cholinesterase inhibitors such as organophosphate and carbamate insecticides, drugs, and a large number of organic phosphorus esters used m industrial processes, has led to the refinement of rapid and reliable means of cholinesterase assay for human and animal blood specimens. Because mild clinical cases of antiche linesterase poisoning may mimic a variety of commonly encountered diseases it is useful to have available reliable tests for assessing the cholinesterase status of patients with histories suggesting possible exposure to cholinesterase inhibitors. Cholinesterase assays of the usual type depend upon the general reaction scheme illustrated in Fig. 1. Hydrolysis of acetylcholine (or other esters of choline) by the enzyme results in the production of free acetic acid. Hydrogen ion arising from dissociation of the acetic acid may be measured by several techniques, some of which arc also listed in Fig. 1. Rate of acetic acid production by the enzyme from the substrate can be quantitated to give an index of the cholinesterase activity in the specimen. Choice of an assay method for toxicological use should depend pnmarily on the demand for cholinesterase testing whch will be met by