We examined the effects of cysteamine and dimethylsulfoxide on the lethality of iododeoxyuridine-labeled HeLa cells irradiated with 150 kV x-rays (effective energy was 48 keV) and compared with those irradiated with 60Co gamma-rays. The protectable fraction was estimated from the regression lines for the reciprocal plots of the concentration of protectors, versus the degree of protection. The protectable fraction by cysteamine was 9.4% lower in iododeoxyuridine-labeled cells than in control cells when they were irradiated with x-rays, whereas no such difference was observed with dimethylsulfoxide. The slopes of the regression lines were steeper (i.e., more difficult to protect) for both protectors in iododeoxyuridine-labeled cells irradiated with x-rays. No such differences were observed in gamma-irradiated cells. The cause for these differences are attributable to Auger enhancement. The results suggest that at least a portion of Auger enhancement is not protectable by cysteamine and is hard to be protected by both protectors cysteamine and dimethylsulfoxide.