Populations derived fromSalmonella typhimuriumsurviving 1, 5, or 10 exposures to 0.5 Mrad did not show any increase in resistance to antibiotic discs containing 5 μg of tetracycline, 5 μg of chloromycetin, or 50 units of polymyxin. The frequency of resistance to 3 μg of tetracycline, 10 units of ampicillin, 120 μg of chloromycetin, or 30 μg of streptomycin per milliliter of agar, did not increase among progeny derived from either of two strains ofS.typhimuriumsurviving 5–10 exposures to 0.5 Mrad. So, too, the virulence and radiation resistance of three strains ofS.typhimuriumexposed to radiation-growth cycling remained essentially unchanged or decreased.A mixture which originally contained 10 different serotypes ofSalmonellawas cycled at 0.23 Mrad. After 10 exposures the frequency of resistance of the progeny to ampicillin, chloromycetin, and streptomycin increased 87-, 55- and 13-fold respectively. The LD50was essentially the same, but the radiation resistance (Dvalue) increased from 0.053 Mrad in controls to 0.074 Mrad.The evidence presented seems to indicate that mutants are attained more readily fromSalmonellacultures recycled at 0.25 Mrad or less than from those recycled at 0.5 Mrad. The pathogenicity of either one was usually essentially unchanged or reduced.