Glutamate dehydrogenase is an enzyme responsible for ammonium assimilation and glutamate catabolism in organisms. The tylosin producerStreptomyces fradiaepossesses both NADP- and NAD-dependent glutamate dehydrogenases. The latter enzyme was purified 498-fold with a 7.5% recovery by a six-step protocol. The enzyme is composed of two subunits, each ofMr47 000, and could form active aggregates of four or eight subunits. Its activity was inactivated by alkaline pH or temperatures of −20 °C or above 40 °C. Activities assayed in the direction of oxidative deamination and reductive amination were optimal at pH 9.2 and 8.8, respectively, and at temperatures of 30–35 °C. No activity was found when NAD(H) was replaced with NADP(H). TheKmvalues were 32.2 mM forL-glutamate, 0.3 mM for NAD+, 3.4 mM for 2-ketoglutarate, 14.2 mM for NH4+, and 0.05 mM for NADH. Deamination activity was partially inhibited by adenyl nucleotides and several divalent cations; amination activity was not affected by the nucleotides but significantly inhibited by Cu2+or Ni2+.Key words:Streptomyces fradiae, NAD-dependent glutamate dehydrogenase, purification, properties.