Ten lots of diced cured ham, packed in cans, were inoculated with approximately 106 Clostridium botulinum spores per can. Each lot was seeded with a different strain (five type A and five type B strains). All cans were irradiated to various dose levels with Co60. Evidence provided by swelling, toxicity, and recoverable C. botulinum with 6,350 cans demonstrated that: (i) 4.5 Mrad was more than adequate as a sterilization dose; (ii) the minimal experimental sterilizing dose (ESD) based on nonswollen nontoxic endpoints was 2.0 < ESD < 2.5 Mrad, and based on nonspoiled sterile cans was 3.0 < ESD < 3.5 Mrad (the latter was supported by the computed theoretical 12D dose); (iii) D values calculated from botulinal survival data indicated that, as a group, the type A strains were more radioresistant than type B strains; strains 12885A and 41B, with respective D values of 0.242 and 0.175, represented the most resistant of each type; (iv) swollen cans did not always contain toxin, nor were toxic cans always swollen; (v) viable C. botulinum can exist for 6 months at 30 C without producing visible or toxic spoilage at doses of 3.0 Mrad and lower, including, in some instances, 0.0 Mrad; and (vi) a phenomenon similar to heat activation of spores occurred at sublethal radiation doses. MATERIALS AND MErHODS Test organisms. Ten strains of C. botulinum were used: 33A, 36A, 62A, 77A, 12885A, 9B, 40B, 41B, 51B, 53B. These strains represent the highest, lowest, and intermediate radioresistances of 102 strains screened in a model system (4). The sources of these organisms, their serotypes, maintenance, and spore preparations were previously described (4). The only modifications employed were the substitution of distilled water for buffer as the spore diluent, and the use of pork-pea-agar (2) in conjunction with screw-cap 166