HSP27 exists as large aggregates that breakdown after phosphorylation. We show rat cardiac HSP27 is S-thiolated during oxidant stress, and this modification, without phosphorylation, disaggregates multimeric HSP27. Biotinylated cysteine acts as a probe for thiolated proteins, which are detected using non-reducing Western blots probed with streptavidin-horseradish peroxidase. Controls show a low level of S-thiolation, which is increased 3.6-fold during post-ischemic reperfusion. S-Thiolated proteins were purified using streptavidin-agarose, and Western immunoblotting showed HSP27 was present. We increased protein S-thiolation 10-fold with 10 mum H2O2 with or without a kinase inhibitor mixture (staurosporine, genistein, bisindolylmaleimide, SB203580, and PD98059). H2O2 alone induced the phosphorylation of HSP27 Ser-86 and Ser-45/Ser-59 of its homologue alphaB crystallin. However, kinase inhibition reduced phosphorylation of these sites below basal. Despite effective kinase inhibition, H2O2 still disaggregated HSP27, but not aB crystallin. This is consistent with the lack of an S-thiolation site on aB crystallin. Thus, we have demonstrated a novel mechanism of HSP27 multimeric size regulation. S-Thiolation must occur at Cys-141, the only cysteine in rat HSP27.