We examined whether fast-neutron radiotherapy is superior to low-LET radiotherapy by comparing the relationship between cell survival and tumor control probabilities after exposure of tumor-bearing (species) to the two modalities. Analysis based on TCD50 assay and lung colony assay indicated that single dose of fast neutron achieved animal cures at higher survival rates than other radiation modalities including single and fractionated gamma-ray doses, fractionated doses of fast neutron, and the mixed-beam scheme with a sequence of N-gamma-gamma-gamma-N. We conclude that fast-neutron radiotherapy cured animal tumors with lower cell killing rates other radiation modalities.