T-T cell interactions during in vitro cytotoxic T lymphocyte (CTL) responses. II. Helper factor from activated Lyt 1+ T cells is rate limiting i) in T cell responses to nonimmunogenic alloantigen, ii) in thymocyte responses to allogeneic stimulator cells, and III) recruits allo- or H-2-restricted CTL precursors from the Lyt 123+ T subset.