The assay of neurotoxic esterase (NTE) in brains taken from dosed hens enables potential neurotoxicity of organophosphate pesticides, plasticers, etc. to be assessed. The original assay [Johnson, M.K. Biochem. J. 114, 711-717 (1969)] has been simplified to eliminate centrifugation and transfer steps and both the selectivity and the sensitivity have been increased. The procedures necessary to obtain stable reagent stocks are described.