1. Leukemic Swiss mice of ICR/Ha strain which had been injected at birth with a lymphoid-leukemia-inducing virus preparation yielded sera which produced elevations of serum colony-stimulating activity within 16 hr and significant plasma-LDH-enzyme elevation at 4 days when injected intraperitoneally into normal ICR/Ha Swiss mice. Colony-stimulating activity was assayed in vitro by the stimulation of hemopoietic colony formation by DBA/1 bone marrow cells.
2. The inducing agent in leukemic serum was passageable, filterable, sedimentable, and heat-, ether-, and UV-labile.
3. A similar agent was recovered from normal Swiss serum after blind serial passages through normal mice.
4. LDH elevating virus induced a similar elevation of serum colony-stimulating activity when injected at high titers, and cross-resistance was demonstrated between LDH virus and the passaged leukemic serum agent.
