The effects of interleukin 2 (IL-2) on In vitro T cell development in organ cultures of day 14 foetal mouse thymus were investigated. Over a 12 day culture period IL-2 was found to inhibit the development of each of the major thymocyte subpopulations, defined by CD4 and CD8 expression. Since each subpopulation was reduced in number, a common precursor to these subsets was thought to be inhibited by IL-2. Indeed, subsequent analyses of subsets of CD4− CD8− cells, characterized by expression of HSA, Pgp-1, and IL-2R, demonstrated that CD4− CD8− cells expressing IL-2R and their postulated progeny were reduced in IL-2-treated cultures. The number of cells reputed to be the immediate precursor of CD4−CD8−IL-2R+ cells, i.e. CD4−CD8−Pgp-1 +IL-2R- cells, was not changed by treatment with IL-2. Interestingly, however, numbers of the most immature subset of CD4-CD8- cells, identified as having the HSA1oPgp-1 +IL-2R− and CD3− phenotype, increased in IL-2-treated cultures and was the only population of cells to do so. We also document the failure to detect any LAK cell activity against syngeneic thymocytes in IL-2-treated cultures and therefore hypo thesize that the inhibition of T cell differentiation in these cultures is due to an arrest in differentiation of CD4−CD8− cells bearing the IL-2R.