The immune response to SRBC was measured in the spleens of adult thymectomized, total body irradiated mice injected with various combinations of thymus and bone marrow cells together with thymic humoral factor (THF). It was found that the number of plaque-forming cells was significantly increased when THF was given in vivo immediately after thymus cell administration or when thymus cells were incubated in THF before injection. On the other hand, bone marrow cells equally treated did not manifest any T cell activity, since THF-treated bone marrow cells were not able to substitute thymus cells in the system used. The results accumulated in the present experiments indicate, therefore, that the target cells for THF activity are thymus cells which acquire a higher T helper cell capacity after THF treatment.

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