Five types of Bacillus subtilis spores differing in DNA repair and recombinational capacities were exposed in vacuum to monochromatic soft X rays from synchrotron radiation. The inactivation rate constants were obtained from exposure-survival curves upon irradiations at 12 wavelengths in the range of 0.1000 nm (12.40 keV) to 0.6000 nm (2.066 keV). Spores of two repair-deficient strains, UVS (uvrA ssp) and UVP (uvrA ssp polA), exhibited almost equal sensitivities to those of wild-type UVR+, while those of two recombination-deficient strains, RCE (recE) and RCF (recF), exhibited higher sensitivities in the whole wavelength range. This suggested that the repair of DNA damage produced by soft X rays was dependent on the recombinational capabilities. Inactivation action spectra based on photon fluence showed that the effectiveness of the radiation increased as the wavelengths became longer. Abrupt changes in the effectiveness occurred around the wavelengths corresponding to the absorption edges of K-shell electrons of phosphorus and calcium. In both cases, the sensitivity was the highest at the wavelengths of the resonance absorption peak, the next highest at those of the higher energy, and the lowest at the lower energy. Mass energy absorption coefficients of spores were obtained from the transmission of a flake made of spores. They were used to derive inactivation action spectra based on absorbed doses. In these spectra, basal levels of the sensitivity seemed constant, and enhancements of the sensitivity were observed consistent with the absorption by calcium and phosphorus. Thus calcium and phosphorus atoms were the predominant targets for the absorption events leading to the inactivation of spores in the wavelength range examined.