Spleen and thymus leucocytes from control Xenopus toadlets react similarly in vitro to classical thymus-dependent (t) mitogens (phytohaemagglutinin (PHA) and Concanavalin A) as measured by enhanced DNA synthesis. The effect of different concentrations of foetal calf serum on the dose response curves to those two mitogens is examined. The effect of sequential thymectomy on PHA responsiveness of spleen leucocytes reveals that thymectomy until stage 55of development (31 days) abrogates PHA reactivity. In contrast, thymectomy at stage 56/7 (35-40 days - just prior to metamorphosis) and in early adult life has no apparent debilitating effect on the response to this mitogen. Optimal conditions for splenic mixed leucocyte reactivity (MLR) in control Xenopus have been investigated. The use of allogeneic serum enhances MLR stimulation indices. The effect of sequential thymectomy on splenic MLR shows that thymus removal only until stage 51 (15-16 days) abrogates this allogeneic response. Thymectomy from stage 52-57 (21-40 days), and in adulthood, failed to abolish MLR. It appears that lymphocytes that can participate in MLR are established in the periphery (e.g. spleen) early in development, in contrast to the slower "emergence" of PHA- reactive cells. These studies support a concept of T cell heterogeneity in amphibians
