The reactivity of thymic lymphocytes in the mixed lymphocyte reaction of the toad Xenopus laevis is already established in the 4th week after fertilization. The degree of tritiated thymidine incorporation by lymphocytes and stimulation indices were of the same order of magnitude during ontogeny. This stability of the thymocyte reactivity contrasts with other aspects of the thymus development. At metamorphosis, the number of lymphocytes in the thymus is reduced by a factor of 5, as compared to the maximum number of cells in larval stage.

The use of a sensitive immunofluorescence technique suggested that a previously noticed decrease in the percentage of immunoglobulin-bearing thymocytes is likely to be due to the lower expression of immunoglobulin molecules at the surface of the cell. None of these ontogenetic changes seemed to affect the reactivity of thymocytes in the mixed lymphocyte reaction, at least in unidirectional reactions between individuals from unrelated families.