OBJECTIVE To determine the cause of an unusual hemoglobin (Hb) pattern detected during HbA(1)c monitoring. METHODS Hemolysate was analysed by ESI MS, and individual components purified by reverse phase HPLC. Peptide mapping was used to pinpoint the substitution and DNA sequencing to confirm the mutation. RESULTS ESI MS of lysate showed three novel β chains with mass changes of -83, -51 and +222 Da. Peptide mapping and DNA sequencing indicated a β37Trp→Cys substitution. Reverse phase chromatography showed three new β globins eluting ahead of β(A) CONCLUSION: The new Hbs result from an initial β37Trp→Cys mutation (-83 Da) followed by oxidation to cysteine sulfinic acid (+32 Da) and the formation of a glutathione adduct (+305 Da). Despite the hydrophobicity change and the critical location of the side chain on the α1β2 interface, there was no evidence of molecular instability or altered oxygen affinity, and no clear phenotype apart from discordant HbA(1)c.