Spontaneous metastases in lymph nodes and/or the lung were obtained after tumour cell inoculation of four mouse tumours and one rat tumour into the foot-pads of syngeneic animals or their F1 hybrids. Following local radiotherapy with doses of 45-80 Gy, significantly more mice died with metastases than following local amputation of the tumour-bearing foot when the 2661 carcinoma was involved. No significant difference was observed after these treatments for the other tumours. The enhancement of metastastic growth after local radiotherapy in the 2661 carcinoma seems not to be due to incomplete killing of tumour cells in the foot. The presence of irradiated normal structures and tumour tissue after radiotherapy promoted the outgrowth of 261 carcinoma cells which were outside the radiation field at the time of treatment. Evidently, even under similar experimental conditions, radiotherapy may enhance the growth of metastases from some tumours and not from others.