The 4.3-kb gene coding for L-lactate dehydrogenase of Bacillus stearothermophilus has been subcloned and expressed in E. coli cells. The enzyme was purified 200-fold with 25% yield by heat treatment , DEAE-Sephadex, and NAD++ -Sepharose CL-4B affinity chromatography followed by gel filtration through Sephadex G-200 . The molecular weight of the purfied enzyme was estimated to be about 35, 000 and 140, 000 on SDS-polyacrylamide gel electrophoresis and gel filtration, respectively. indicating that the enzyme is composed of four identical subunits. THe enzyme for pyruvate reduction and lactate oxdiation was stable at 60 and 75$^{circ}C$ for 30 min, and the optimal temperatures for both reactions were 60 and 7$0^{circ}C$, respectively. The enzyme had an optimal pH at 5.5 and 8.5 in pyruvate reduction and lactate oxidation, respectively. The pH stability of enzyme of pyruvate reduction was table between pH 5 and 7. more than 90% of enzyme activity was lost at 1mM FeSO4 and p-chloromercuribonzoate. The maximal activation of the enzyme was obtained with 0.8mM fructose 1, 6-bisphosphate.