The radiosensitization of Escherichia coli and Salmonella typhi in presence of carvacrol (1.0%) and tetrasodium pyrophosphate (0.1%) in ground beef packed under four different atmospheres (air; 100% CO 2; MAP : 60% O 2–30% CO 2–10% N 2 and vacuum) was determined. Medium fat ground beef containing carvacrol and tetrasodium pyrophosphate was inoculated with E. coli or S. typhi. Samples were packed under different atmospheres as described before and irradiated at doses from 0.1 to 0.6 kGy in presence of E. coli and from 0.50 to 2.0 kGy for S. typhi. Results indicated that the best increase in radiosensitization was observed in samples packed under MAP. The D 10 values were 0.046 kGy for E. coli and 0.053 kGy for S. typhi. Both bacteria were more resistant to irradiation under air, in the absence of active compound. A D 10 values of 0.126 kGy for E. coli and 0.526 kGy for S. typhi was observed.