A biologically pure RNAIII inhibiting peptide (RIP), that includes five contiguous amino acids of the sequence YX.sub.2PX.sub.1TNF, where X.sub.1 is C, W, I or a modified amino acid, and X.sub.2 is K or S is provided. The RIP further includes amino acids having a sequence that differs from the sequence YX.sub.2PX.sub.1TNF by two substitutions or deletions, where X.sub.1 is C, W, I or a modified amino acid, and X.sub.2 is K or S. This agent offers improved protection against and treatment of staphylococcal infections, and related bacteria infections, in mammals.