Larvae of the South African clawed toad, Xenopus laevis, were thymectomized at Nieuwkoop and Faber stages 45-46 (4-5 days after fertilization). The 1-1.5-year-old thymusless toads thus obtained were implanted with histocompatible thymus from 1-year-old toads, and 4 weeks later were given skin allografts or rabbit red blood cells (RRBC) to determine their immune response. In contrast with the perfectly tolerant thymusless controls, the thymus-implanted toads rejected skin allografts after longer survival times (25-66 days) than the nonthymectomized controls (18 - 19 days). Accelerated rejection (15 - 27 days) of the second-set skin allografts was evident in the thymus-implanted individuals. Similarly, the capacity of antibody production against RRBC was restored to the thymus-implanted toads to roughly the normal extent. Implantation of thymus also restored the histological features of the spleen in thymectomized toads. The experiments clearly demonstrate that in Xenopus the thymus plays an essential role for immunity at the early stages of its histogenesis. The restorative capacity of the adult thymus is also stressed.