Glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate dehydrogenase (GDH), alanine dehydrogenase (ADH) and alanine aminotransferase (GPT) were detected in the cell-free homogenate of Streptomyces avermitilis grown in a defined medium containing ammonium sulfate as the only nitrogen source. At an initial NH4+ concentration of 7.5 mmol/L, high activities of GS, GOGAT and GDH were found while that of ADH was low. The ADH activity was markedly increased at initially millimolar NH4+ concentrations. In some characteristics of its NH4+-assimilating system (e.g. control of some enzyme activities, the NADPH specificity of GOGAT, the presence of alanine aminotransferase), S. avermitilis differs from other known streptomycetes. © 1992, Folia Microbiologica. All rights reserved.