The development of spontaneous lymphoma in a high leukemia strain of mice was inhibited by administration of cortisone, although the hormone-induced atrophy of the thymus and other lymphoid tissues was only transient.

Inhibition of lymphoma formation resulted from only three successive daily injections of cortisone but was greater if the cortisone administration was prolonged for three months.

A high incidence of myeloid leukemia was induced in a low-leukemia strain of mice by a single exposure to 350 r of x-radiation.

Cortisone was without effect on the induction of myeloid leukemia.

Cortisone administered preirradiation increased, and postirradiation decreased, the incidence of radiation-induced lymphomas in male mice.

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