alpha/beta-Type small acid-soluble proteins (SASP) bind to spore DNA and protect it against ultraviolet light, heat, hydrogen peroxide and freeze drying, making the spores much more resistant than vegetative cells to these agents. Spores of a mutant of Bacillus subtilis lacking the two major alpha/beta-type SASP were almost 30 000-fold less resistant to hypochlorite than were wild-type spores. After treatment with hypochlorite, surviving spores of the mutant, but not those of the wild type, showed higher levels of mutation, suggesting that SASP contribute to hypochlorite resistance by protecting spore DNA.