The invertase of Lactobacillus reuteri CRL 1100 is a glycoprotein composed by a single subunit with a molecular weight of 58 kDa. The enzyme was stable below 45 degrees C over a wide pH range (4.5-7.0) with maximum activity at pH 6.0 and 37 degrees C. The invertase activity was significantly inhibited by bivalent metal ions (Ca++, Cu++, Cd++, and Hg++), beta-mercaptoethanol, and dithiothreitol and partially improved by ethylenediaminetetraacetic acid. The enzyme was purified 32 times over the crude extract by gel filtration and ion-exchange chromatography with a recovery of 17%. The K-m and V-max values for sucrose were 6.66 mM and 0.028 mu mol/min, respectively. An invertase is purified and characterized for the first time in Lactobacillus, and it proved to be a beta-fructofuranosidase.