The effects of thymectomy and thymus graft restoration upon the in vitro primary responses to alloantigens and PHA have been studied. It has been found that neonatal thymectomy substantially eliminates both PHA reactivity and responsiveness to alloantigens assayed in vitro in host spleen cell populations. Analysis of albumin density gradient-separated subpopulations of the spleen and thymus in such animals was also performed. It was found that the total and proportional representation of the individual density subpopulations was identical in neonatally thymectomized, in normal, and in thymectomized and thymus graft-restored animals. Therefore, thymectomized mice appear to retain a nonfunctioning, small, dense, lymphocyte population. Reconstitution of thymic-dependent in vitro reactivity was nearly complete when syngeneic, but not allogeneic or semisyngeneic thymus was employed. Occasional partial restoration did occur when F1 thymus was employed, but never when allogeneic thymus was grafted. The grafted thymus contained PHA and alloantigen-reactive cells in a large, less dense B layer subpopulation, whereas the restored animals, as in the case of normals, showed these reactivities to be a property of a small, more dense cell population.

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