Syngeneic tumour cell suspensions were injected into the foot or intravenously in normal rats or rats bearing subcutaneous tumours. Resistance was assessed by measuring the swelling of the foot due to the growth of the tumours or by counting lung colonies or metastases. The tumours included two methyl-cholanthrene-induced fibrosarcomas (D7, D8), a metastatic variant of D8 (D8M) and a spontaneous adenocarcinoma (ST-2). D8, D8M and ST-2 induced concomitant immunity to challenge in the foot. D8 and ST-2, but not D8M, induced concomitant immunity in the lung. Concomitant immunity was, in each case tested, non-specific and could be expressed against a tumour which itself failed to induce concomitant immunity. Irradiation significantly enhanced the growth of D7, D8 and ST-2 in the feet of non-immune rats. Irradiation, carrageenan and silica significantly reduced the growth of D8M in the feet of non-immune rats. Irradiation increased the growth of D8 and D8M and carrageenan increased the growth of D8 in the feet of concomitantly immune rats (i.e., depressed resistance). Carrageenan decreased lung colony formation by D8 in normal and concomitantly immune rats and silica decreased colony formation by D8 in concomitantly immune rats (i.e., increased resistance). Thymus-deprived rats allowed greater growth of D8 in the feet but still expressed concomitant immunity. It is concluded that: (1) both natural and acquired tumour immunity may be modulated non-specifically in rats; (2) concomitant immunity, which has a large non-specific component, may contribute to the suppression of metastases but may also have site specificity; (3) the exact nature of the rat host-tumour relationship differs from tumour to tumour.