A bacteriocin, streptococcin sal-P, was isolated by freeze-thaw elution from cultures of Streptococcus salivarius strain P grown on tryptic soy agar plus 1% neopeptone. The inhibitor could also be extracted with either 7 M urea of 1 M NaCl from cells grown on this medium, but little activity was recovered from cells grown in liquid media of from the supernatants of these cultures. Streptococcin sal-P was found to be a proteinaceous substance of molecular weight approximately 8000. It was remarkably stable at extremes of pH or temperature and appeared to adsorb nonspecifically to both sensitive and resistant bacterial cells and also to cellulose membranes. The range of its inhibitory activity was almost entirely against Gram-positive bacteria, particularly streptococci, including strains of S. pyogenes and S. sanguis, but not S. mutans or group D streptococci. Streptococcin sal-P was bactericidal for actively metabolizing susceptible strains.