Microcin 7 is an antibiotic peptide, produced and excreted to the culture medium by E. coli strains harboring the plasmid pRYC7. This peptide was extracted from the culture media by adsorbing it on octadecyl silica. It was purified by gel filtration on Sephadex G-25 and reverse phase high performance liquid chromatography. Its amino acid composition is the following: Ala (0.8), Arg (1.9), Asx (1.9), Gly (1.5), Met (0.8) and Thr (0.9). The purified peptide does not react with ninhydrin and it is resistant to carboxypeptidase degradation, indicating that the molecule may be a cyclic or end-blocked oligopeptide.