SUMMARYUsing a modified parental-F1hybrid, graft-versus-host, method, attempts were made to promote the maturation of potential immunologically competent cells of fetal liver origin (parental) in thymectomized F1hybrid hosts by means of irradiated and nonirradiated parental and allogeneic thymus grafts, suspensions of cells from parental thymus, and parental and allogeneic thymus placed in diffusion chambers. It was found that, in this experimental model, only those thymus grafts which survived in the primary host for a prolonged period (i.e. parental) and to which the fetal liver cells had direct physical access would promote the maturation of these lymphoid precursor cells. In addition it was noted that: (1) the methods used had immunologic specificity; (2) viable fetal liver cells or thymus grafts were required in all experimental systems; (3) dissociated thymus from 18–20-day embryos gave no evidence of the presence of either mature or immature lymphoid cells; and (4) potential immunologically competent cells appeared to be incapable of producing hemagglutinins to rat erythrocytes or of rejecting allogeneic or xenogeneic skin grafts in the absence of the host thymus.