A T-cytotoxic response was demonstrated in a non-productive, viral-induced syngeneic tumoural system in vitro, but it did not result in tumour rejection: When tumour cell inoculum was removed surgically early after transplantation and processed, a specific T-cell-mediated cytotoxic response was observed in vitro and in the Winn assay; however, a tumour-cell challenge was rejected, showing that under certain conditions, the immune response can modify tumour evolution. Localized irradiation was administered according to one of two protocols (7 Gy X 3 over a week and 4 Gy X 3 over 3 weeks). The cumulative dose of the latter regimen was nearly 20% greater than that of the former; however, the outcomes of the two therapies were dramatically different: 50% survival in group 2, and a short delay in tumour growth in group 1. The scattered dose (of around 0.15 Gy) delivered to the spleen in the first group decreased the T-cytotoxic response, but that in the second group (scattering inferior to 0.07 Gy) did not. The similarity in therapeutic results in the two groups when the irradiation regimens were preceded by immunosuppression suggests that this failure of the immune response could influence the outcome of radiotherapy. The effect of low doses of irradiation on tumour rejection was also studied in an allogeneic system.