1. A 33 week old Swiss female mouse that had been caged with AKR males exhibited a "spontaneous" lymphoblastoma resembling the spontaneous disease in the AKR strain.

2. Tumor cell suspensions of this animal did not grow when inoculated into AKR mice.

3. Tumor cell suspensions grew progressively in both Swiss and DBA mice, producing a leukemia.

4. Cell-free filtrates of leukemic Swiss or DBA mouse brain induced leukemia in 48 of 106 adult Swiss mice inoculated intracerebrally and 39 of 55 animals inoculated intraperitoneally. The leukemia appeared in the majority of the animals within one to three weeks from the time of inoculation.

5. Cell-free filtrates of the leukemic tumor failed to induce leukemia under the conditions of the experiment.

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