A patient receiving antineoplastic (and immunosuppressive) chemotherapy for metastatic carcinoma of the uterine cervix developed a respiratory illness similar to an illness in her pet dog. The patient and her three adult children all had a history of very close contact with the dog, and both dog and humans harbored Mycoplasma canis, a canine mycoplasma, in their throats. The dog and all of the humans had some serologic reaction to this organism. Colonization by M. canis of humans in close contact with a dog was documented, but disease due to infection remains to be proven.