An investigation has been made of the development of various T cell functions in lethally irradiated mice reconstituted with anti-0 treated spleen or bone marrow cells. Evidence is presented to show that both organs contain a post-thymic precursor pool able to regenerate by 15 days limited T cell responses in thymectomized recipients. A prethymic pool also exists in each organ able to regenerate, at a later date, first a suppressor T cell population and probably later, mature functional T cells involved in helper functions and cell mediated lympholysis. The spleen is apparently a better source of precursors of the suppressor cells than bone marrow, while a poorer source of precursors of the other T cell functions. All T cell functions investigated apparently first appear in large cells which undergo a reversion to small cells without necessarily maturing to their full potential reactivity. By following the kinetics of appearance of T cell functions, and the physical parameters of the cells with which these functions are associated, it is shown that PHA responding and Con A responding cells, cytotoxic T cell progenitors, helper T cells for antibody production and helper T cells for cytotoxicity induction can all at some stage of differentiation be separated from one another.