Ginkgo biloba extract(GBE), extract from leaves of Ginkgo biloba tree, is known to increase peripheral blood flow. Under the assumption that GBE would increase tumor blood flow and improve oxygen supply to the tumor and enhance radiatian sensitivity, the effect of GBE on radiation tumor growth delay was investigated. C3H mouse fibrosarcoma(FSaII), 6 mm in size, growing in the hidleg muscle of C3H mouse was used. GBE was injected i.p. once 1 hr prior to irradiation or twice 25 hrs and 1 hr prior to irradiation. Tumor growth was not influenced by GBE treatment alone. Tumor growth delay by a single dose of 50 mg/kg and l00 mg/kg of GBE plus 10 Gy of radiation was prolonged to 1.23 times and to 1.36-1.49 times when compared with 10 Gy irradiation alone. Tumor growth delay by two doses of 100 mg/kg of GBE plus 10 Gy of irradiation increased to 1.66-1.84 times(p<0.05). Radiation dose required for 3 day tumor growth delay decreased to half when two doses of 100 mg/kg of GBE was given prior to irradiation. Based on these results, it is concluded that GBE enhances radiation sensitivity of C3H mouse fibrosarcoma(FSaII) and could be a potential radiosensitizer for the malignant tumor.