We have synthesized a novel stable precursor, saligenin phosphorotrichloridate, which, on reaction with N-monobiotinyldiamines, generates a series of biotinylated covalent inhibitors of serine esterases. A homologue designated S9B [1-(saligenin cyclic phospho)-9-biotinyldiaminononane] was selected to allow detection and rapid isolation of neuropathy target esterase (NTE). This enzyme is the primary target site for those organophosphorus esters (OPs) which cause delayed neuropathy. NTE comprises about 0.03% of the total protein in brain microsomal fractions and has resisted purification attempts over many years. S9B is a potent progressive inhibitor of NTE esteratic activity (second-order rate constant 1.4 x 10(7) M-1.min-1). Incubation of S9B with brain microsomes led to specific covalent labelling of NTE as determined by detection of a biotinylated 155 kDa polypeptide on Western blots. Specificity of S9B labelling was further demonstrated by inhibition with the neuropathic OP mipafox. Biotinyl-NTE in SDS-solubilized S9B-labelled microsomes was adsorbed on to avidin-Sepharose and subsequently eluted, yielding a fraction enriched approx. 1000-fold in NTE by a single step with recoveries of 30%. Essentially pure NTE was obtained after separation from two endogenous biotinylated polypeptides (120 and 70 kDa) in avidin-Sepharose eluates by preparative SDS/PAGE. Other biotinylated saligenin phosphoramidates derived from the same precursor may be useful for detection and isolation of other serine esterases and proteinases.