Reactions of illudin S with cysteine derivatives (cysteine methyl ester, glutathione and a peptide, Cys-Asp-Pro-Gly-Tyr-Ile-Gly-Ser-Arg) were investigated. In the reaction with cysteine methyl ester, four products (P1, P2, P3, P4) were obtained and their structures were determined, on the basis of MS and NMR data, to be adducts of the mercapto group of cysteine methyl ester with the alpha,beta-unsaturated carbonyl group of illudin S. In the reactions with glutathione and the peptide, two addition products in each case were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and NMR analyses. The structures of these adducts also indicated that the alpha,beta-unsaturated carbonyl group in illudin S behaves as a Michael acceptor for the mercapto group in cysteine.