The effect of 1 g of ascorbic acid, administered intravenously to healthy male and female volunteers, on blood and its constituents was investigated by means of electron spin resonance (ESR) spectroscopy and by differential hematologic examinations. The native blood ESR spectrum exhibits 2 min injection of vitamin C a considerable increase in spin concentration and a new signal at about g = 2.005 which we previously had found to correlate to the semidehydroascorbate radical. This spectrum is identical to that obtained in acute lymphatic leukemia. While it prevails in untreated leukemic patients, spin concentration and shape of the spectrum obtained return to normal within several hours to one day in healthy individuals. Since neither in erythrocytes nor in plasma modifications could be observed, the ESR changes detected must have their origin in the leukocytes. Hematologic studies of the peripheral blood show that the number of granulocytes, lymphocytes, and thrombocytes was not drastically affected by the vitamin C injection.