Release of thymidine from x-irradiated spleen in the rabbit as determined by a new bioassay.

of bone marrow cells from untreated rabbits. The factor responsible for the decreased uptake was found to be dialyzable and, on paper chromatography in 4 different solvent systems, had the same RF values as did thymidine. Serial dilutions of this factor gave a dose-related response in the bone marrow suspensions that was identical to that obtained with thymidine, 0.01 to 0.25 ¿ig/ml,confirming the Chromatographie identification of the plasma factor as thymidine. The extent of thymidine release as measured by this sensitive method may be useful in evaluating early cell damage following X-irradiation.