The phytase activity of 12 species of sourdough lactic acid bacteria was screened. It was intracellular only, largely distributed among the species and strains of Lactobacillus sanfranciscensis possessed the highest levels of activity. A monomeric ca. 50-kDa phytase was purified to homogeneity from L. sanfranciscensis CB1 by three chromatographic steps. L. sanfranciscensis CB1 exhibited the highest hydrolysing activity on Na-phytate after reaching the stationary phase of growth (ca. 12 h). Cells cultivated in the presence of maltose and fructose showed an increase of the phytase activity of ca. 35% with respect to the other carbon sources used. The phytase was optimally active at pH 4.0 and 45 jC. The enzyme was strongly inhibited by 2 mM of phenylmethylsulfonyl fluoride (PMSF), and 2 mM Hg 2 + and Fe 2 + . It had a pI of ca. 5.0. The substrate specificity was dependent on the type of phosphate ester; a very low activity was detected on a-D-glucose-1-phosphate and D-fructose-6- and 1,6-phosphate, while the highest hydrolysis was found towards adenosine-5V-tri-, di- and mono-phosphate. Compared to these substrates, the activity on Na-phytate was also relevant. The enzyme was thermo-stable after exposure to 70 jC for 30 min; the D value calculated at 80 jC was ca. 10 min. As shown by the Central Composite Design (CCD) applied to study the individual and interactive effects of pH, temperature and NaCl, acidic conditions and elevated temperatures were indispensable for the enzyme adaptation to high NaCl concentrations. L. sanfranciscensis CB1 cells or the correspondent cytoplasmic extract were used to ferment a sourdough for 8 h at 37 jC; a marked decreased (64 – 74%) of the Na-phytate concentration was found compared with the unstarted dough. The sourdough started with L. sanfranciscensis CB1 cells was re-used for several times and the phytase activity was maintained to a considerable level.