Recombinant human interleukin-5 exists as four major isoforms all possessing N-terminal methionine. Peptide mapping and subsequent analysis by fast-atom-bombardment mass spectrometry (f.a.b.-m.s.) have shown that N-terminal modifications are the cause of the charge heterogeneity. In order of decreasing abundance, these are unmodified methionine, retention of N-terminal formyl group, oxidation of N-terminal methionine to sulphoxide and carbamoylation of the N-terminus. These results were confirmed by analysis of the reduced and alkylated intact protein by electrospray-ionization mass spectrometry. The implications of these findings for the production and characterization of recombinant proteins are briefly discussed.