T cell-deficient adult X. laevis can be generated by thymectomy (Tx) at an early larval stage before stem cell immigration (review in Horton et al., 1998b). Tx adults have dramatically impaired anti-tumor responses (Robert et al., 1997) and skin allograft immunity (Horton et al., 1998b). In Tx animals, peripheral T cells (PBL and splenocytes) are absent whereas the percentage of B cells (Hsu & Du Pasquier, 1984) and NK cells (Horton et al. 2000) are increased; in vitro responses of Tx splenocytes to phytomitogens or alloantigens are also abrogated (Horton et al., 1998). Tx adult frogs are instrumental in assessing the importance of T cells in X. laevis anti-tumor and anti-viral immunity.