In the experiments presented, Virus myxomatosum was observed to produce only a localized fibromatous or myxomatous orchitis when injected into the testicles of cottontail rabbits. This type of disease was quite unlike the acute fatal illness which the virus caused in domestic rabbits. 10 serial passages of Virus myxomatosum through cottontail rabbits, covering a total elapsed time of 140 days, failed to alter its pathogenicity for domestic rabbits. Although it proved impossible to convert the myxoma virus into fibroma virus by serial passage in cottontail rabbits, it was found that these animals, recovered from myxoma, had a solid resistance to infection with the fibroma virus. Furthermore, their sera possessed neutralizing antibodies effective against the fibroma virus as well as Virus myxomatosum. A similar cross-immunological relationship was observed in the cases of domestic rabbits that had survived an attack of infectious myxoma.

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