The efficacy of surgery alone or combined with preoperative radiation was examined using isotransplants of the spontaneous fibrosarcoma (FSaII) growing in the right leg of syngeneic C3Hf/Sed mice and employing as end-points both local tumor control probability and normal tissue damage. Complete assays have been performed on small (4-5 mm) and large (8 mm) tumors. The TCD50s (the dose required to control half the irradiated tumors) for radiation alone were 63.2 and 82.7 Gy for small and large tumors, respectively. Surgery was performed 4 days after the radiation treatment. Surgical resections were performed under an 8 X dissecting microscope. In small tumors, radical (enbloc resection) and conservative (local resection) surgery alone achieved 69.8% (30/43) and 21.9% (7/32) tumor control. The corresponding figures for large tumor were 25% (8/32) and 6.1% (2/33), respectively. The combination of radiation and surgery increased tumor control frequency up to 100% with doses lower than TCD50 for radiation alone. The TGF is expressed as therapeutic gain factor for conservative and for radical surgery for 4-5-mm and for 8-mm tumors from analyses of dose response curves for tumor control and leg shortening. In small tumors, the TGFs at the 80% tumor control (TCD80) and leg shortening of 5 mm were 1.4 and 1.8 for radical and conservative resection, respectively. The TGFs at the TCD50 level for large tumors were 1.4 and 1.7 in respective resections. In this particular experimental system, conservative surgery yielded higher TGFs than radical surgery for both tumor sizes. No effect of preoperative treatment on the frequency of distant metastasis was observed.