Tobacco leaves inoculated with 108 cells/ml of the incompatible bacteria Pseudomonas pisi, P. syringae, P. lachrymans, or Erwinia amylovora developed hypersensitive tissue necrosis, on the first day after inoculation. Similar tissue necrosis developed on the leaves inoculated with the compatible bacterium P. tabaci on the second day. By the time the symptoms developed ammonia gas had evolved from the inoculated leaves. Tissue necrosis caused by bacteria could be reproduced by exposing healthy tobacco leaves to ammonia gas. The amount of ammonia evolved during the development of either the hypersensitive reactions or the disease was enough to account for the formation of tissue necrosis in tobacco.