1. Cell-free suspensions and filtrates were prepared from human leukemic and nonleukemic brains and were injected into adult Swiss mice.
2. The suspensions prepared from the leukemic brains, when concentrated by high-speed centrifugation, produce leukemia in about 15 per cent of the inoculated mice.
3. When the same sediment is combined with heat-inactivated sediment, the incidence of induced leukemia increases to 40 per cent; when combined with hyaluronidase, to about 50 per cent.