Primary cytotoxic responses of DBA/2 lymph node cells to a syngeneic tumour (the mastocytoma P815) have been generated in vitro. The development of these responses is dependent on the addition of a soluble factor (CSCS) which is produced by concanavalin A-activated spleen cells. The response is mediated by T lymphocytes, can be detected at low effector to target cell ratios and is directed against P815 tumour-associated antigens.