The susceptibility of spores of Bacillus subtilis, Bacillus cereus, and avirulent Bacillus anthracis to treatment with hydrogen peroxide, peroxyacetic acid, a peroxy-fatty acid mixture, sodium hypochlorite, and acidified sodium chlorite was investigated. Results indicated that B. cereus spores may be reasonable predictors of B. anthracis spore inactivation by peroxyacetic acid-based biocides. However, B. cereus was not a reliable predictor of B. anthracis inactivation by the other biocides. In studies comparing B. cereus and B. subtilis, B. cereus spores were more resistant (by 1.5 to 2.5 log CFU) than B. subtilis spores to peroxyacetic acid, the peroxy-fatty acid mixture, and acidified sodium chlorite. Conversely, B. subtilis spores were more resistant than B. cereus spores to hydrogen peroxide. These findings indicated the relevance of side-by-side testing of target organisms and potential surrogates against categories of biocides to determine whether both have similar properties and to validate the use of the surrogate microorganisms.