The effect of administering various vehicles on the response to radiation of SCCVII tumors in C3H mice was investigated. When saline, 0.5% carboxymethyl cellulose solution, or 1% hydroxypropylmethyl cellulose (HPMC) solution was given orally 30 min before single 15 Gy irradiation, the tumor regrowth was significantly faster than that seen after 15 Gy treatment alone. There was no difference in this radioprotective response due to the type of vehicle. On the other hand, the tumor regrowth was similar to that seen after 15 Gy alone when saline was given intravenously 20 min before irradiation or intraperitoneally 30 min beforehand, or when 1% HPMC was given orally 2 h beforehand. Oral vehicle administration shortly before irradiation can cause radioprotection of murine tumors, probably by increasing the hypoxic fraction.