1. Intermittent cortisone therapy given over a period of 59 weeks caused permanent granulocytosis in mice from a nonleukemic Albino strain. The granulocytosis persisted for the lifetime of the animals, the longest survival being 97 weeks. Two of 40 animals developed histologic findings entirely comparable to human myeloid leukemia.
2. It is assumed that such cortisone therapy produces a permanent change in the basic mechanisms which control the production of granulocytes or their release from the bone marrow. Possibly the metabolic change may be related to inhibition of vitamin B6 activity.