Mineralization of bacterial spores with Ca2+ and a variety of other mineral cations enhances resistance to heat damage. Part of the enhancement is associated with increased dehydration of the mineralized protoplast or spore core, while part is independent of dehydration and effective for resistance even to dry heat. Spore mineralization was found also to enhance resistance to oxidative damage caused by agents such as tertiary butyl hydroperoxide or H2O2. In contrast, mineral cations in the environment increased oxidative damage, presumably by catalyzing radical formation. Metal ion chelators such as o-phenanthroline protected spores against such damage.