The cDNA coding for amino acid residues 121-269 of human interleukin 1 beta was cloned and expressed at Sclavo Research Center by C. Baldari et al. (1987, EMBO J. 6, 229-234) using Saccharomyces cerevisiae as the host cell. The purified protein showed a very low specific activity when compared to that of a mature recombinant interleukin expressed from Escherichia coli. Preliminary experiments indicated the occurrence of post-translational events in the yeast-derived protein. In an attempt to correlate the structural modifications and the biological activity of recombinant interleukins, the two proteins were characterized by fast atom bombardment mass spectrometry (FAB/MS) following the FAB-mapping procedure. The amino acid sequence of interleukin expressed in E. coli was identical to that expected whereas the mass spectrometric analysis of the recombinant S. cerevisiae protein confirmed the occurrence of covalent modifications. In particular, the asparagine residue at position 7 (numbering follows the mature active protein sequence) was shown to be glycosylated and the two cysteine residues at position 8 and 71 were involved in an S-S bridge. The results demonstrate the value of FAB/MS in the quality control of recombinant interleukins.