The limited differentiation of mature T cell function from mouse bone marrow in tissue culture is described and compared with similar differentiation occuring in vivo in irradiated bone marrow protected mice. Data are presented to show that a pool of precursors, similar in size to that able to produce early (transient?) regeneration in thymectomized recipients, is responsible for the development of mitogen responsive T cells active in MLC (proliferation) and CML (development of cytotoxic cells) assays. In contrast, a helper cell population which augments antibody formation from T-depleted normal spleen cells derives from a pool of similar precursors yet does not seem to be theta positive. Similarly, larger cells (perhaps typical of those giving rise to suppressor T cells in vivo) give rise to a suppressor cell pool after 4 days of culture, though again only a fraction of this suppressor activity could be attributed to theta positive cells. It is suggested that much of the data for regenration of T lymphocytes in vitro from T-depleted sources needs to be re-interpreted in terms of this evidence for a pool of post-thymic precursors of T cells in such T-deficient cell populations.