Kim, K. (University of Washington, Seattle), and N. B. Groman. Mode of inhibition of diphtheria toxin by ammonium chloride. J. Bacteriol. 90:1557-1562. 1965.-The inhibition of diphtheria toxin by ammonium salts was independent of toxin concentration over a 100-fold range of toxin. Inhibition by minimal concentrations of ammonium chloride was abolished by lowering the pH, indicating that free ammonia is the active form of inhibitor. A single addition of ammonium chloride inhibited toxin for a limited period of time, but periodic readdition of the ammonium salt was required to sustain inhibition indefinitely in the absence of antitoxin. Toxin was not destroyed and its adsorption occurred equally well in the presence or absence of ammonium chloride. Preadsorbed toxin was also effectively inhibited by the addition of ammonium chloride. Inhibited toxin remained accessible to antitoxin neutralization. Attempts to reverse ammonia inhibition by the addition of succinate or reduced nicotinamide adenine dinucleotide were unsuccessful. Attempts to inhibit toxin by interfering with active transport were also unsuccessful.