Growth control of experimental tumor by syngeneic thymocyte extracts

The effect of cell-free extracts obtained from thymocytes of 8, 18 and 23 day fibrosarcoma-bearing U rats and normal U rats on tumor growth was investigated. The results show that thymocyte extracts of 8 day tumor-bearing rats inhibit the tumor growth in implanted animals and no significant effect was found with extracts from other times, or from normal thymocytes. Some of the inhibitory activity was lost by dialysis, which led us to assume that low molecular weight substances are involved in the antitumor immune response.