Suspensions of Corynebacterium ovis subjected to physical and chemical treatment failed to kill inoculated mice but were able to produce sterile pyogenic lesions. It appears that the exotoxin is different from the pyogenic factor and that the latter is attached to a heat-stable substance in the bacterial cell. Mice immunised with antitoxin showed local non-spreading lesions when subsequenly inoculated with washed C ovis cells while unprotected control mice showed a spreading type of infection when similarly injected. Antitoxin did not prevent the formation of pus but hindered the spread of infection from the site of inoculation to the internal organs.