A new bacteriocin produced by Aerococcus viridans was purified. Bacteria grown on liquid medium synthesized a bacteriocin, viridicin, which can be extracted from the cells by treatment with 0.86 M NaCl solution. Viridicin production was not induced by ultraviolet irradiation or by treatment with mitomycin C. The bacteriocin was purified by ultrafiltration, gel filtration, and preparative polyacrylamide disc gel electrophoresis. The molecular weight was between 100,000 and 120,000. The purified viridicin was homogeneous on polyacrylamide gel electrophoresis. The viridicin was composed of protein, carbohydrate, and lipid.