Dextranase activity was determined in cell extracts and cell-free filtrates of Streptococcus mutans strains which had been grown in batch culture. Exo-dextranase activity was located chiefly in cell extracts, whereas endo-dextranase was mainly extracellular. Release of endo-dextranase began early in the exponential phase of growth, and ended when the concentration of residual sugar was low. Thus, dextranase expression was associated with rapidly growing cells, the yield of dextranase was increased several fold when the initial concentration of D-glucose in the medium was changed from 0.5% to 2%. The endo-dextranase was not stable at pH 5, and control of the pH of the culture was essential to preserve active dextranase during overnight growth. Strain Ingbritt (serotype c) and serotype d strains were the best dextranase producers; other strains (serotypes a, b, c, e and f) displayed much lower activity. The ability to produce endo-dextranase, and to synthesize alpha-D-glucans with a high proportion of (1 leads to 3)-linked sequences, appeared to be related properties. The possibility is discussed that the release of two enzymes, namely endo-dextranase and the D-glucosyltransferase (GTF-I) that synthesizes (1 leads to 3)-alpha-D-glucan, are factors that contribute to the cariogenicity of S. mutans serotype d.