An hepatic disease caused by a filterable agent carried in leukemic mice is described. Ordinarily the virus remains latent and asymptomatic. If, however, the mice are treated with urethane or methylformamide before and after virus inoculation, the disease becomes manifest and is characterized by extremely marked liver necrosis.

Infant mice, a large percentage of weanlings, and adult Bagg albino mice are killed when injected with a filtrate from organs of diseased animals. Adult F1 and Swiss mice show signs of the disease but generally recover. They succumb, however, when simultaneously treated with urethane or methylformamide. By continuing the treatment of consecutive transfer generations an acute disease can be induced which finally kills all adult F1 mice without the treatment. At this stage the original leukemia may be lost.

Mice which have recovered from the subacute disease are resistant to the acute disease, and mice injected with the latent form of the agent are immune to the subacute disease. However, even immunized animals lose their resistance if they are treated with urethane. The acute or subacute disease can be reduced to the latent stage by passing the agent through several generations of immunized animals.

The relationship of this hepatitis virus of mice to viruses causing similar diseases is discussed, as is the possibility that these agents are closely related, if not identical.

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