1. The physiological actions of HCN and its salts appear to be due (a) to the ease with which HCN molecules penetrate living cells and then ionizing, exert their influence by means of H ions and CN ions; (b) to the weakness of HCN as an acid, which permits at neutrality or at even slight alkalinity the presence of a considerable amount of free HCN molecules in the presence of their salts; (c) to specific effects occasioned by its chemical activity. 2. The order of resistance of various protozoans to HCN resembles that of the same protozoa to CO2 and to H2S, but is the reverse of their resistance to mineral acids. 3. HCN in acid, neutral, or slightly alkaline media produces intracellular acidity because of the rapid penetration of HCN molecules into the cell. 4. HCN acts specifically on certain species of protozoa, the order of resistance of Paramecia being Paramecium caudatum > Paramecium mullimicronucleatum > Paramecium aurelia > Paramecium bursaria.