The reversible binding constant (Ki) for tetramethylammonium ion (TMA) was determined from the decrease in the bimolecular rate constant (ki) observed with each of 21 organophosphate or carbamate inhibitors of acetylcholinesterase (EC 3.1.1.7). The Ki values obtained were reasonably constant (5.8 X 10(-4) +/- 0.38 M), and this is consistent with reports indicating that TMA binds to a single site on the enzyme.