The effect of rifampicin after irradiation with electrons or neutrons on cell-killing, DNA degradation and the ability of DNA-membrane complex extracted from irradiated bacteria to synthesize DNA was studied in strains E. coli B/r and BS minus 1. Post-irradiation treatment with rifampicin greatly enhanced the lethal effects of both electrons and neutrons on the repair-proficient strain E. coli B/r under anoxic conditions. However, the same post-irradiation treatment of the repair-deficient strain E. coli BS minus 1 with rifampicin gave no consequent increase in lethality. Rifampicin did not alter the extent of DNA degradation in either strain after irradiation with electrons or neutrons. The DNA-synthetic activity of DNA-membrane complex from electron-irradiated E. coli B/r and BS minus 1 was not altered by post-irradiation treatment with rifampicin, nor did this treatment affect synthesis by DNA-membrane complex isolated from neutron-irradiated BS minus 1; but the complex, isolated from E. coli B/r which had been irradiated under anoxic conditions with neutrons, showed a marked decrease in activity. We suggest that post-irradiation sensitization with rifampicin of the repair-proficient E. coli B/r, especially after irradiation under anoxic conditions, indicates that initiation of the synthesis of an RNA molecule is involved. In addition our results suggest that the structure of the membrane complex in E. coli B/r and BS minus 1 is different.