It was found that concentrated polyethylene glycol 400 (PEG 400) solutions have significant antibacterial activity against various pathogenic bacteria, including Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus. This effect might be attributed to two effects: lowering of water activity and, superimposed on this, the specific action of PEG-400 molecules on bacterial cells. Phase-contrast microscopic observations of cells placed in contact with PEG 400 revealed clumping and morphological changes of bacterial cells. The larger changes in appearance were evidenced by the species which were more rapidly killed by PEG 400. The results obtained suggested that concentrated PEG 400 solutions may have a potential value in medicine as a topical antibacterial agent. The feasibility of this application is the subject of present investigation.