The ability of crude extracellular enzyme produced by the bacterium Streptococcus mutans AL-7 to lysis the cells of five species, sixteen strains, of oral streptococci was examined. This enzyme showed lytic activity of living cells of only Streptococcus sanguis ATCC 10558 strain, and of heated cells of Streptococcus sanguis ATCC 10558 and ST-7 strains. The long chains of Streptococcus sanguis ATCC 10558 was severed at random by this enzyme. Early log phase cells of this strain were more sensitive to this enzyme than late log phase cells. No bacteriocins or bacteriocin-like substances were produced by Streptococcus mutans AL-7 strain in chemically defined medium. In view of these results, the relationship between this lytic enzyme from Streptococcus mutans and a decrease in the number of serotype III strains of Streptococcus sanguis in dental plaque is suggested.