reportcd the presence of a G leukeniogenic agent, presumably of viral nature, i n certain inbred strains of mice with a high incidence of spontaneous lymphoid leukemia. Inoculation oE cell-free extracts from leukemic tissues of such donors transmitted leukemia to inbred strains of mice with :t low incidence of the disease at any age. InocuLition of newborn recipients with the leukemic extracts was essential for leukemia induction. This work was fully confirmed by otlicrs.fl~ 11, 1x9 21.22 Rapid serial passage of the leukcmogenic extracts through mice of low leukemic strains ultimately yielded a highly active virus inoculum, the "Passage A" virus.10 This potentiated inoculum produced leukemia not only in newborn but also weanling and young adult mice after a considerably shortened latent period. In these experiments, the mechanism oI leukemia ti ansmission has been refcrred to as induction. In contrast, acceleration of leukemia refers to a process by which an expected disease, such as the "genetically determined" lymphoid Icukemia of the AKK strain appears earlier alter various experi-tuc~ntrll procedures. Accclcration of lymphoid leukemia in the high leukemic AKR strain by means of isologous leukemic extracts was first described by Rudali, nu plan, and Latarjet.15~ 16 Newborn AKR mice were inoculated with cell-free filtrates and cerilrifugites obtained from leukemic tissues of adult AKR donors. As a result, the latent period of leukemia was