Glutaminase from Micrococcus luteusK-3 [Micrococcusglutaminase (Mglu); 456 amino acid residues (aa); 48 kDa] is a salt-tolerant enzyme. Our previous study determined the structure of its major 42-kDa fragment. Here, using new crystallization conditions, we determined the structures of the intact enzyme in the presence and absence of its product l-glutamate and its activator Tris, which activates the enzyme by sixfold.