Human and mouse granulocyte-macrophage-colony-stimulating factors (hGM-CSF and mGM-CSF, respectively), isolated from Escherichia coli cells expressing the corresponding human and mouse genes, have been characterized. The observed properties of the proteins have been compared with those properties which can be deduced from the DNA sequence alone and the published properties of natural GM-CSFs. The purified E. coli-derived proteins were found to have the expected molecular masses, amino acid compositions and N- and C-terminal amino acid sequences. The finding of 70-90% unprocessed N-terminal methionine for both proteins is discussed. The four Cys residues were found to be involved in two intramolecular disulphide bonds, linking the first and third, and second and fourth Cys residues. This disulphide bond arrangement is probably the one existing in natural material, since, although not glycosylated, both E. coli-derived proteins showed biological activity (colony stimulating assay for hGM-CSF, and cell proliferation assay for mGM-CSF) comparable with that reported for the respective proteins purified from animal cells.