An enzyme termed organophosphorus hydrolase (OPH), derived from Pseudomonas diminuta, had been found previously to hydrolyze the powerful acetylcholinesterase (AChE) inhibitor O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothiolate (VX). This enzyme has now been shown to be correlated with the loss of AChE inhibitory potency (detoxication). OPH also hydrolyzed and detoxified the VX analogue, O,O-diisopropyl S-(2-diisopropylaminoethyl) phosphorothiolate (Tetriso), also a potent AChE inhibitor, about five times faster than VX. The Km for the hydrolysis of the P-S bond of Tetriso was 6.7 x 10(-3) M. OPH also hydrolyzed diisopropylphosphorofluoridate (DFP) 50-60 times faster than Tetriso, and 1,2,2-trimethylpropyl methylphosphonofluoridate (Soman) about seven times faster than Tetriso. DFP was a non-competitive inhibitor of Tetriso hydrolysis, Ki = 8.7 x 10(-4) M. The DFP hydrolysis product, diisopropyl phosphate, was a competitive inhibitor, Ki = 2.3 x 10(-4) M. The rate of detoxication of Tetriso compared with the rate of hydrolysis suggests that OPH may not be totally specific for P-S bond cleavage. OPH was inhibited completely by 1.5 x 10(-4) M 8-hydroxyquinoline-5-sulfonate or 1,10-phenanthroline, both transition element chelators, but inhibited only partially by EDTA, a much more potent chelator.