A strategy is described to raise high-affinity antibodies directed against the organophosphorus nerve agent VX [O-ethyl S-(2-diisopropylamino)ethyl)methyl phosponothionate]. Ten chemical derivatives of VX (haptens) have been synthesized. Their structures differ principally from VX structure by substitution of S-atom by an O-atom or CH2-group and by introduction of a reactive group (carboxylic acid, arylamine or primary amine) on the O-ethyl side chain. None of these haptens, except one, exhibit potential toxicity as tested by their inhability to inhibit acetylcholinesterase (E.C. 3.1.1.7.). After coupling with a protein carrier, they were injected intradermally to rabbits. Nine of these immunogenic conjugates led to the appearance of antibodies able to bind VX in a competitive solid phase immunoassay. The apparent titer and affinity of the antisera differed greatly depending on the hapten used. The highest affinity (9 nM) was observed with the VX derivative bearing O-S substitution and O-ethyl-carboxylic side chains. The antibodies appear specific for VX, since cross-reactivity with other nerve agents (Soman, Sarin or Tabun) was low. However, two haptens elicited antibodies with affinity to Soman or Sarin in the micromolar range. Antibodies were able to neutralize VX inhibition of acetylcholinesterase in vitro but not in vivo.