By means of an assay of graft-versus-host activity some properties of the thymic humoral factor which confers immunocompetence upon lymphoid cells in vitro have been studied. Allogeneic and xenogeneic thymic preparations were found to activate lymphoid cells from neonatally thymectomized mice, enabling initiation of a graft-versus-host response. Thus, this thymus factor is apparently neither strain nor species specific.
The active principle of calf thymus extracts was found to be in the supernatant after prolonged ultracentrifugation. When exhaustive dialysis and ultrafiltration through Diaflo membranes were performed, the active thymus agent was found to pass through both the dialysis sac and Diaflo UM-2 membranes. The molecule which confers immunocompetence upon lymphoid cells thus seems to be of molecular weight of an order of magnitude of 1000 or less. Dialyzed thymus preparations injected into neonatally thymectomized mice also restored the capacity of spleen cells of these mice to induce graft-versus-host activity. When injected into intact mice, thymus extract also increased the proportion of competent cells in the spleens of these animals, probably by activation of target cells originating outside the spleen.