Intracellular invertase from Streptococcus mutans GS5 was purified to near homogeneity by gel filtration and ion-exchange chromatography followed by preparative polyacrylamide gel electrophoresis. The invertase appeared to be composed of a single polypeptide chain with a molecular weight of 48,000. Extracellular invertase was identified in strain GS5 and was determined to have a molecular weight of 500,000. No antigenic relationship between these two forms of invertase was observed since antibody prepared against purified intracellular invertase neither affected extracellular invertase activity nor precipitated that enzyme on immunodiffusion. No antigenic relatedness between intracellular invertase and glucosyl- and fructosyl- transferases was detected since cross-reactivity with antibody prepared against either enzyme fraction was not observed after immunodiffusion. Using immunodiffusion and quantitative precipitin data, we examined the relationships of other S. mutans intracellular invertases to the serotype c enzyme. It appeared that the intracellular invertases from serotypes e, f, and g were structurally similar to the enzyme from serotype c, whereas the structure of invertases from serotypes a, b, and d appeared less similar to that of enzyme from serotype c.