A single I. P. injection of evaporated bovine milk or sodium caseinate into mice immediately after a whole body exposure to 600 R of x-rays increased the 30-day survival from 45 to 73 and 80%, respectively. These substances also doubled the mitotic activity of bone marrow in nonirradiated and irradiated (300 R) mice within 6 hours of injection. In contrast, a single intraperitoneal injection of calcium caseinate and bovine parathyroid extract (given subcutaneously) were without effect on either the mitotic activity of bone marrow or the level of radiation mortality. The bovine parathyroid hormone did not affect the levels of the total or free calcium in the blood, while milk and calcium caseinate increased the total calcium concentration, but did not affect the concentration of physiologically active free calcium. Sodium caseinate, on the other hand, transiently lowered the levels of both the total and the free calcium. Evidence is presented which suggests that a hypocalcemia-induced release of endogenous parathyroid hormone is responsible for the mitogenic and therapeutic activities of sodium caseinate. The mechanism of action of milk is not known. The mitogenic and therapeutic actions of milk and sodium caseinate support the concept that the postirradiation stimulation of cell proliferation in hematopoietic tissues can play an important role in reducing radiation lethality.

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