The properties of DNA structure and the phospholipid content of Salmonella derby cells were studied with respect to their plasmid content and radiosensitivity. The role of R-plasmid in determining the qualitative and quantitative compositions of S. derby phospholipids was revealed. The radiosensitivity of plasmid-carrying S. derby mutants was shown to be most likely determined by the structure of DNA, its GC content, and the level of methylation. We suggest that the phospholipid molecules and their interaction with DNA play a key role in formation of the radio-resistance of plasmid-free S. derby cells.