Larval cells of the anuran tadpole are replaced by adult cells in the corresponding tissues of the frog during metamorphosis: tissues of the tail, which have no counterpart in the adult, are completely eliminated during metamorphosis. We have previously demonstrated that young adults of the major histocompatibility complex (MHC) homozygous inbred J strain of Xenopus laevis reject skin grafts from larvae of the same strain, showing that there is histoincompatibility between larval and adult skin tissues [19]. Thus, we postulated that some immunological recognition might be involved in this specific elimination of the tail tissue and set out to test the idea. Using in vitro assays, we detected a significant increase in proliferation of splenocytes derived from adult and metamorphic climax animals co-cultured with larval tail tissue. This response was shown to be thymus-dependent. However, the degeneration of larval tissues was not observed in such co-cultures under our standard culture conditions. To detect the possible cytotoxicity of splenocytes, the culture conditions were modified by supplementing with 10% heat-inactivated adult Xenopus serum instead of 10% fetal calf serum (FCS). After this modification, the degeneration of larval tissues was observable macroscopically and microscopically with cocultured adult splenocytes, but not tadpole ones. The nuclear fragmentation of the epidermal cells was seen by light and electron microscopy. Apoptosis was evidenced by the demonstration of the ''ladder pattern'' upon electrophoresis of genomic DNA obtained from the degenerating larval tissues. Surprisingly, this response was thymus independent. Moreover, it was shown that this response was not observed when the larval tissues were cultured with adult thymocytes or adult epidermal cells. In vivo, migration of T cells into the epidermis of tail tissues at the late climax of metamorphosis was demonstrated immunohistochemically using a monoclonal antibody against Xenopus T cells, even in the early thymectomized tadpoles. Considering these results, we propose that populations of adult-type non-T leukocytes might participate in the specific detection and elimination of larval type cells during metamorphosis.