Materials and Methods Materials. Chlorpyrifos oxon was kindly provided by Dow Chemical Co.(Midland, MI). Spectral analysis indicated little contamination by 3, 5, 6-trichloro-2-pyridinol. Paraoxon was obtained from ICN K & K Labs (Plainview, NY). Material from other suppliers was found to contain unacceptable levels of p-nitrophenol. Caution, these organophosphates are highly toxic nerve poisons. They have potent anticholinesterase action and can be absorbedpercutaneously. Therefore, they should be handled with great care. Waste from the assays can be inactivated by strong NaOH solutions. Cibacron Blue 3GA-agarose type 3000 was from Sigma Chemical Co.(St. Louis, MO). DEAE-Trisacryl M was from IBF Biotechnics (Savage, MD). Immobilon P membranes were obtained from Millipore Corp., San Francisco, CA.