Murine T cell lines that help induction and generation of allospecific cytotoxic T cells (CTL) from thymocytes cultured together with allogeneic stimulator cells have been established. It was done by successively culturing T cell blasts obtained from mixed lymphocyte cultures (MLC) in the medium supplemented with T cell growth factor (TCGF). The helper function of the long-term cultured T cells was antigen specific, radiation resistant, and not H-2 restricted. The helper function has been retained for more than 8 mo by the cells continually proliferating in the TCGF-supplemented medium. The T stimulated responsiveness of thymocytes to T cell mitogens. CTL activity seen in the original MLC waned rapidly during successive culture of this T cell line in TCGF medium T cell blasts from MLC successively cultured in continual presence of TCGF and the stimulator alloantigens retained strong CTL activity against the allogeneic stimulator cells for about 3 mo. All these T cell lines proliferating in TCGF medium expressed Thy 1 antigen and showed the appearance of a large T cell blast. These T cell lines could not proliferate in the absence of TCGF and lost responsiveness to T cell mitogens intrinsically.