Highly purified calf thymus preparations were obtained by sulfuric acid extraction procedure. These were further purified by ion-exchange chromatography on a CM-cellulose column. Among the preparations thus obtained, a 0.1 N HCl eluted fraction (CM-3) showed such distinct biological activities as promotion of lymphocytopoiesis, mainly of small lymphocytes in neonatally thymectomized mice and the protective modification of radiation injuries. As control samples, the equivalent spleen and liver preparations, 2-aminoethylisothiuronium (AET), and bovine serum albumin were used in these experiments. Although there was no definite organ or substance specificity in survival tests, thymus but not spleen and liver extracts were effective. On bone marrow nucleated cellularity, CM-3 showed marked protection in comparison with that injected with an equivalent dose of AET. The effect of this purified thymus preparation was also tested on small intestine disaccharidase activities of lethally irradiated mice. The enzymatic activities, based on the measurement of maltase and trehalase, were protected by the treatment, indicating that the test might be useful in evaluating the efficiency of radioprotective agents in animals irradiated with high dose level. It is suggested that the biological activities of thymic preparations shown in the present studies are associated, in some way, with thymic histone in those sulfuric acid extracts.

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