Abstract
A filterable agent designated "virus X" recovered from a C3H female in which leukemia was induced by X-ray irradiation, was passed serially through 4 consecutive cell-free inoculations of suckling C3H mice. Of 40 mice inoculated, 26 thus far developed leukemia (65%) at average ages varying from 3 to 11 months. Inactivated (56[degree]C, 1/2 hour) rabbit and guinea pig serum, prepared with agent X filtrates, only partially neutralized passage A leukemic agent, whereas passage A immune serum had a distinct, though not complete neutralization effect on the passage A agent. The passage A agent had been previously reported (Gross, L.: Proc. Soc. Exptl. Biol. and Med. 94: 767-771, 1957); it originated from spontaneous Ak leukemia and has been passed by cell-free inoculations in newborn or suckling C3H mice.

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