Aspartate aminotransferase from Lactobacillus murinus is thermostable, its activity being not changed for two months at temperatures between 4 and -70 degrees C. Maximum activity was observed at 40 degrees C and pH 7.3 in phosphate buffer (30 mmol/L). delta G* Value of 26.3 kJ/mol was calculated from the Arrhenius plot. The Km values for L-aspartate and 2-oxoglutarate at pH 7.3 were 25 and 100 mmol/L, respectively. Sodium maleate and glutamate acted as inhibitors of the enzyme activity. The Ki values for sodium maleate with L-aspartate of 2-oxoglutarate as variable substrates were 1.1 and 0.5 mmol/L, respectively. The Ki values for glutamate with L-aspartate or 2-oxoglutarate were 8.0 and 4.0 mmol/L, respectively. An inhibitory effect was observed with 1 mM Hg2+ ions (1 mmol/L). The activity of the enzyme was diminished by only 12% in the absence of pyridoxal 5'-phosphate.