Two weeks after grafting, whole thymus implants had the characteristic appearance of a normal young thymus. They increased in weight until 6 weeks post-operation and then underwent involution. Maximum weights attained by grafts in male hosts were significantly higher than those attained in female hosts. Initial rates of graft involution were higher in males than in females. Weights attained by grafts in young hosts of either sex reached maxima that were significantly higher than those attained by grafts of the same age in old hosts. The number of thymuses grafted separately into a single host did not influence the weight of the host's own thymus or the mass of individual grafts. However, when multiple grafts were linked together on transplantation, they behaved as an individual graft with respect to general morphology, growth rate and maximum size. With respect to growth and involution, it is suggested that thymic lymphopoiesis is mainly regulated by intrinsic factors emanating from the tissue which survives implantation. Further, the regulators appear to be clone-specific.