The cDNA encoding for Mus musculus myoglobin (Mb) was amplified using standard RT-PCR techniques and cloned in an appropriate bacterial expression vector. For the first time, mouse Mb was recombinantly expressed in Escherichia coli cells, BL21(130), and purified in sufficient amounts to carry out a preliminary characterization. As shown by mass spectrometry, the protein is found in complex with glutathione, which binds the Cys residue in the topological position E9, in the proximity of the heme group. In recombinant murine Mb, azide affinities are only slightly dependent on the Cys(E9) oxidation state. This suggests that Cys(E9) does not provide a relevant contribution for, the: stabilization of ligands bound to the heme iron atom. Recombinant expression of M. musculus Mb might have an important role in order to investigate the eventual involvement of Cys(E9) in the new physiological roles proposed for Mb. (C) 2003 Elsevier Science (USA). All rights reserved.