A standard procedure for lipid-extraction of lyophilized hen brain material is described. Nine carboxylesterase isoenzymes (EC 3.1.1.1) are identified in lipid-extracted lyophilized material (LELM) using kinetic analysis of organophosphate inhibition. Total phenyl valerate (PV) hydrolysing carboxylesterase activity in LELM is 43.3 U X g-1. Two carboxylesterase isoenzymes of LELM are classified as neurotoxic esterases (NTEA and NTEB). Using n-octylglucoside 51% of the water-insoluble neurotoxic esterase activity from LELM are solubilized. Six carboxylesterase isoenzymes including NTEA (6.5 U X 1(-1] and NTEB (4.2 U X 1(-1] are present in the solubilized preparation. Throughout purification and separation steps carboxylesterase isoenzymes are identified by their rate constants for the reaction with organophosphorus inhibitors.