Mutanolysin partially purified from the culture filtrate of Streptomyces globisporus 1829 consists of two main lytic enzymes with an isoelectric point near pH 8.5 and 10, respectively, and proteolytic enzyme is associated with the latter lytic enzyme. Mutanolysin exhibited maximal lytic activity at 60 C in the pH range 6.5 to 7.0 and was stable at 50 C in the acid range. N-bromosuccinimide caused complete inhibition of lytic activity at 1 mM, whereas calcium and magnesium ions at the same concentration caused activation. Mutanolysin had lytic or bactericidal activity against the living cells of Streptococcus mutans, Streptococcus salivarius, Streptococcus sanguis, Lactobacillus acidophilus, and Actinomyces viscosus, which are considered to be etiologic agents of dental caries, but had no activity against S. aureus and all gram-negative strains tested. The lytic activity was well retained in human saliva. Digestion of the cell walls of S. mutans BHT by mutanolysin was accompanied by the liberation of free amino groups and reducing sugars. Mutanolysin may be expected to be a useful agent for dental caries control.