beta-Galactosidase was isolated from the cell-free extracts of Lactobacillus crispatus strain ATCC 33820 and the effects of temperature, pH, sugars and monovalent and divalent cations on the activity of the enzyme were examined. L. crispatus produced the maximum amount of enzyme when grown in MRS medium containing galactose (as carbon source) at 37 degrees C and pH 6.5 for 2 d, addition of glucose repressing enzyme production. Addition of lactose to the growth medium containing galactose inhibited the enzyme synthesis. The enzyme was active between 20 and 60 degrees C and in the pH range of 4-9. However, the optimum enzyme activity was at 45 degrees C and pH 6.5. The enzyme was stable up to 45 degrees C when incubated at various temperatures for 15 min at pH 6.5. When the enzyme was exposed to various pH values at 45 degrees C for 1 h, it retained the original activity over the pH range of 6.0-7.0. Presence of divalent cations, such as Fe2+ and Mn2+, in the reaction mixture increased enzyme activity, whereas Zn2+ was inhibitory. The Km was 1.16 mmol/L for 2-nitrophenyl-beta-D-galactopyranose and 14.2 mmol/L for lactose.