Exposure of albumin to sulfite in the presence of Co(II) or peroxidase/H2O2 caused site-specific fragmentation, which was not due to cleavage of methionyl nor tryptophanyl peptide bonds. The reaction of GlyPro with sulfite in the presence of Co(II) or peroxidase/H2O2 led to Gly liberation, suggesting the oxidative cleavage of protein at Pro residues. Sulfite plus Co(II) induced bityrosine production, Trp loss and a new Trp-derived fluorescence. ESR-spin trapping method provided evidence for the formation of sulfate radical (SO4.-) during Co(II)-catalyzed autoxidation of sulfite. The order of reactivity with SO4.- seemed to be Trp greater than GlyPro greater than GlyGly approximately Gly approximately Pro. The results suggest that SO4.- plays an important role in fragmentation and modification of albumin.