The lactic acid bacterium, Lactobacillus casei, produces an intracellular beta-glucosidase when grown on Man-Rogosa-Sharpe (MRS) medium with cellobiose as carbon source. The beta-glucosidase activity is produced intracellulary, and no extracellulary activity was detected. The enzyme was purified by ion-exchange chromatography and gel filtration. The molecular mass of the purified intracellular beta-glucosidase as estimated by gel filtration was 480 kDa, consisting of six probably identical subunits. The enzyme exhibited optimum activity at 35 degrees C and pH 6.3 with citrate-phosphate buffer. The enzyme was active against soluble glycosides with (1-->4)-beta configuration and from Linesweaver Burk plots, K-m value of 16 mmol/L was found for beta-pNPG. The beta-glucosidase was competitively inhibited by glucose, and no glycosyl transferase activity was observed in the presence of ethanol.