Evidence is presented that the 19S alpha-2 globulin fraction, isolated from human serum or from sheep spleen, is capable of strongly enhancing regeneration of the mouse thymus damaged by radiation injury. This activity is preceded by enhancement of regeneration of the damaged bone marrow. The conditions for maximum activity of this protein fraction were studied in C57BL/6 mice submitted to four weekly irradiations of 170 R. Enhanced regeneration was obtained when each of the four injections of 19S alpha-2 globulin was given several hours before or after each irradiation, with a sharp optimum time of 2 hours before each irradiation. Increasing the number of injections beyond four did not achieve any demonstrable further increase in effect.

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