SUMMARY:Sodium benzylpenicillin, contaminated withBacillus subtilisspores by freeze‐drying a suspension of spores in an aqueous penicillin solution (c. 50% w/v), was exposed to gamma radiation and a 70‐tube dilution method was used to determine the surviving spores after various doses. The correlation coefficient between log10percentage survival and dose was −0.9523. The regression of the former on the latter was calculated and the decimal reduction dose found to be 20.2 × 104rads. The regression and the decimal reduction dose were similar to those obtained when suspensions of spores in distilled water were irra