1. The subcutaneous administration of a single massive dose of cortisone to mature male albino mice produces: (1) A prompt and prolonged fall in the level of circulating lymphocytes and eosinophils which persists as long as nine days after the administration of the hormone. (2) A granulocytopenia which resolves at two days and is accompanied by a period of bone marrow blocking followed by resumption of delivery and myeloid hyperplasia.
2. No inhibitory effect of cortisone on cell proliferation is seen in the bone marrow.
3. There is no change in hemoglobin concentration following cortisone treatment.
4. Adult male albino mice are satisfactory animals for serial observations of the effect of cortisone on the circulating blood elements and the bone marrow.