One of the problems involved in regional perfusion for cancer chemotherapy is protection of the bone marrow outside of the perfused area due to "leak" of the antitumor agent from the perfused area. Since in vitro experiments have shown that reduction in incubation temperature below 37 C depresses the antitumor effect of certain alkylating agents against experimental animal tumors,4 it became desirable to determine if reduction of body temperature would protect the hematopoietic system of the animal against lethal effects of an alkylating agent. Method Paired, healthy mongrel dogs were lightly anesthetized with pentobarbital (Nembutal) anesthesia. The body temperature of one of the paired animals was reduced from 37 C to 31-33 C by immersion in an ice-water bath, and the animal was then removed and allowed to rewarm spontaneously at room temperature of approximately 27 C. A 3-4 cm. segment of rib from each of the 2 animals