KHT tumor [fibrosarcoma] inoculated to the footpads of syngeneic C3H mice spontaneously metastasized-primarily to the popliteal lymph node and secondarily to inguinal and lumbar nodes following a definite latent period. Lymph node weight was used to determine extent of metastasis to popliteal, inguinal and lumbar lymph nodes. Localized X-irradiation of the foot prior to tumor inoculation enhanced the extent of lymphatic metastasis to popliteal, and under certain conditions, inguinal and lumbar lymph nodes. The extent of enhancement was dependent on dose of irradiation and the interval of time between X-irradiation and tumor inoculation. Lung colony counts were elevated in a qualitatively similar manner. Possible mechanisms and implications for radiotherapy are discussed.