Mycoplasma pulmonis has substantial DNase activity exposed on the cell surface. At least part of this activity is attributable to an endonuclease. The activity is destroyed at 56 degrees C and inhibited by either 5 mM EDTA or 10 mM zinc chloride. It can also be eliminated by treatment of intact organisms with trypsin and is regenerated by incubation of the treated organisms in a medium that supports protein synthesis. DNase exposed at the cell surface constitutes 20% of the total DNase activity present in M. pulmonis extracts.