Radiation directed to the spleen reduces the circulating leukocytes in leukemic patients. For further investigation of this phenomenon, 7 patients with chronic leukemia were given infusions of plasma obtained before (control) and after (experimental) they had received radiation to the spleen. The control plasma, as expected, did not lower the leukocyte count; in 2 instances it initiated a progressive increase in the leukocyte level, which continued for several weeks before a plateau was reached. In all but one patient experimental plasma significantly decreased the number of leukocytes. This effect was detectable 12 hours after infusion, reached a maximum in 36 hours, and was dissipated by the fourth day. It is postulated that a humoral factor liberated by the spleen in response to x-irradiation caused this decrease, although the underlying mechanism is unknown.