This paper describes the differentiation of fetal thymocytes in organ cultures of embryonic thymuses. After one week in organ culture, fetal thymocytes from 14 d old mouse embryos could secrete IL-2, CSF and IFN upon stimulation with Con A. No constitutive secretion of lymphokines was observed. The only type of CSF produced was granulocyte-macrophage (GM) CSF. In contrast to thymocytes from adult mice, organ-cultured embryonic thymocytes did not secrete IFN-gamma, but IFN alpha/beta, in response to Con A. This is the first indication that secretion of IFN alpha/beta, but not IFN-gamma, can be induced in cells of the T cell lineage by a T cell specific mitogen. These results show that the embryonic thymus provides a sufficient environment for the development of some of the secretory functions of T cells and suggest that differentiating T cells acquire the ability to secrete IFN alpha/beta before IFN-gamma.