A cloned T cell hybridoma (123) was shown to produce T cell growth factor (TCGF). Supernatants of cultures of hybridoma-123 that had been stimulated with concanavalin A caused T cell blasts or a cloned T cell line to proliferate, allowed a mitogenic response to concanavalin A by thymocytes and by lymph-node cells depleted of accessory cells by treatment with anti-Ia serum and complement, and permitted the generation of both polyclonal and antigen-stimulated cytotoxic T lymphocytes in cultures of thymus cells. These observations suggest that the hybridoma is producing a factor with an identical spectrum of activities to that associated with TCGF derived from mitogen-stimulated spleen cells and indicate that the T cell is the source of TCGF. Together with other evidence that the same hybridoma produces activity affecting B lymphocytes, myeloid progenitor cells, and pluripotential stem cells, these experiments confirm the role of the activated T cell as a regulator of hemopoietic and lymphoid systems. T cell hybridomas should prove invaluable for biochemical and genetic analysis of these factors.