Epoxyquinol B (EPQB) is a fungal metabolite, containing two alpha,beta-epoxy ketones. We previously showed that EPQB inhibited the signal transduction involved in angiogenesis through the binding to cysteine residues of receptor kinases. However, the inhibitory mechanism was undefined. In this report, we found that one EPQB molecule is covalently bound to two L-cysteine molecules through two epoxide residues on EPQB. Furthermore, EPQB crosslinked binding proteins through the cysteine residues. These results suggest that EPQB inhibits receptor kinases by crosslinking with other protein or by intramolecular crosslinking.