Viridicine, a new bacteriocine, was extracted from whole cells of Aerococcus viridans by repeated washings with Tris-HCl 5 X 10(-3) M pH 8,1, NaCl 1 M. A crude preparation of the macromolecule was not inactived by heating for one hour at 50 degrees C and was sensitive to the action of protease, lipase and catalase. The molecular weight was about 100,000. A large number of Gram-negative and Gram-positive bacteria were sensitive to viridicine action.