Murine interleukin-6 (mIL-6) was expressed in Escherichia coli in the insoluble fraction of cell lysates. Approximately equal amounts of two polypeptide species, reactive with anti-IL-6 antibodies, were produced. The two forms of mIL-6 were isolated and found to have identical N-terminal sequences initiated by Met-Phe-Pro-Thr-Ser-Gln-. Peptide mapping after endoproteinase glu-C digestion led to isolation and characterization of the C-terminal peptides from each of the two forms and allowed the source of the heterogeneity to be identified as a C-terminal addition of three amino acids, Gln-Lys-Leu, to authentic mIL-6. Inspection of the nucleotide sequence of the plasmid containing the mIL-6 gene and expression of the plasmid in other strains suggested that the addition of three amino acids was caused by a readthrough of the termination codon arising from an unexpected suppressor mutation in the original host strain. Although the C-terminus of IL-6 is critical for the activity of this cytokine, the IL-6 variant with extended C-terminus was fully active in two separate bioassays. This suggests that the additional amino acids do not disrupt the structure or function of this important region of the molecule.