Neurotoxic esterase is the putative site of initiation of organophosphorus-induced neuropathy. The protein is membrane-associated and will thus require solubilization before it can be purified. Its enzymic activity was retained in hen brain microsomes suspended in 10-60% (v/v in water) dimethylsulfoxide and 5-20% dimethylacetamide, but lost in 5-20% 1- and 2-propanol as well as higher concentrations of dimethylsulfoxide. Soluble activity (100,000 x g, 60 min supernatant) was not obtained with dimethylacetamide, but 24% of the recovered activity (67%) was solubilized in 40% dimethylsulfoxide, with retention of its native response to inhibitors. Solvent extraction of active enzyme is of intrinsic interest and expands the options for its purification.