Four isocyanates, two of which are widely used in industry, were found to be potent inhibitors of acetylcholinesterase from human erythrocytes. Aliphatic isocyanates are stronger inhibitors of the enzyme than are aromatic isocyanates. Incubation at 21 to 23 °C for several days leads to slow and limited spontaneous reactivation of the inhibited enzyme. This enzyme inhibition may be a contributing factor in the induction of respiratory disease observed in about 5% of workers exposed to isocyanate vapors.