It has been suggested that extracts of feverfew may inhibit platelet behaviour via effects on platelet sulphydryl groups. In the present study we have obtained evidence for such a mode of action. Compounds that contain sulphydryl groups such as cysteine and N-(2-mercaptopropionyl)glycine prevented the inhibition of platelet behaviour by feverfew. Feverfew and parthenolide (one of the active components of feverfew) dramatically reduced the number of acid-soluble sulphydryl groups in platelets. This effect occurred at concentrations similar to those that inhibited platelet secretory activity. Feverfew itself did not induce the formation of disulphide-linked protein polymers in platelets but polymer formation occurred when aggregating agents were added to feverfew-treated platelets. Feverfew evoked changes in the metabolism of arachidonic acid that were similar to those observed in glutathione-depleted platelets.