High ionic strength is expected to enhance dissociation of Ca-DPA from spores and to contribute to a detrimental effect on spore stability or on spore boat resistance with a combined treatment of gamma-radiation. From this study, this hypothesis has become apparent as as follows; 1) Ca-DPA dissociation contributes to logs of stability of bacterial spores with respect to heat resistance, survival during storage, and 2) the cytoplasmic membrane plays a role in maintaining the stability of DPA-Ca-spore complex, apparently by serving as a permeability barrier.