Cell-mediated immune reactions against carcinogen-induced rat tumours were demonstrated by the in vitro microcytotoxicity test. Lymph-node cells from syngeneic rats immunized against individual aminoazo-dye-induced hepatomas or 3-methylcholanthrene-induced sarcomas were cytotoxic for cells of the immunizing tumour compared with the effects of normal lymph-node cells. Only a low level of cross-reactivity was detectable against target cells derived from otherr tumours. Conversely, lymph-node cells from tumour -bearing rats were cross-reactive against a range of tumour cells, suggesting that the tumour-bearer was sensitized against a common antigen(s) as well as the individually specific antigen associated with these tumours. Lymph-node cells were reactive at an early time (from day 2) after tumour implantation and their cytotoxic potential persisted until terminal stages of tumour growth. Spleen cells from tumour-bearer and tumour-immune donors also exhibited comparable cytotoxic reactivities of the same specificity as that shown by lymph-node cell preparations. These findings are discussed in relation to the development of cell-mediated immune responses in the tumour-bearing rat and in immunized rats which are capable of rejecting transplanted tumour cells.