A carbene-generating 14C-labelled beta-methoxyacrylate derivative ((E)-methyl-3-methoxy-2-[4-(3-trifluoromethyl-3-diazirinyl) benzoyloxyphenyl] propanoate, uniformly labelled with 14C in the benzene ring of the benzoyl group) has been used to locate the proteins involved in binding this class of inhibitors to bovine heart mitochondrial ubihydroquinone:cytochrome c oxidoreductase. The beta-methoxyacrylate photoaffinity label was shown to be a competent inhibitor of electron transport through the protein complex. Under illumination through a narrow bandpass filter, allowing specific photolysis of the diazirine group, the compound bound to cytochrome b and weakly to an 8 kDa polypeptide. Apart from some binding to a cytochrome b aggregate, other proteins were left unlabelled. The binding could be prevented in the presence of excess amounts of unlabelled beta-methoxyacrylate, myxothiazol or stigmatellin but not by antimycin A or HQNO. At high concentrations DBMIB partially competed for the binding site. The binding site for this class of inhibitors is therefore the 'o-site'. Our results indicate that this site is comprised of residues from cytochrome b and possibly the 8 kDa polypeptide and that the site may be close to the Reiske iron-sulphur protein.