The oxygen enhancement ratio, as estimated after the effect of 137Cs-gamma-quanta, depends on the repair genotype of E. coli K-12 cells and increases in the studied strains in the following order: recA-uvrA(-)----recA(-)----wild type----pol A-. These variations are levelled with the effect of fast neutrons of division spectrum (0.75 MeV); the oxygen enhancement ratio for the strains under study decrease, while the oxygen effect is virtually absent in recA-uvrA--mutant.