The modification of a mercury electrode with 6-mercaptopurine (6MP) has been performed at pH 6 in acetate buffer and the electrode behaviour has been checked in the absence of the modifier in solution. Direct electrochemistry of cytochrome c has been observed on a 6MP-Hg modified electrode. At high coverage of 6MP, the monolayer acts as an effective promoter for the redox exchange of cyt c, which is not observed on a bare mercury electrode. The film is stable for more than 200 scans and must play a role on avoiding irreversible adsorption and denaturation of the protein. At pH 7, 25 mM phosphate buffer and 0.1 M NaClO4, cyt c exhibits a quasi-reversible electron exchange similar to that reported with gold modified electrodes, the electron rate constant being 4.1 x 10(-3) cm s(-1). As far as we know it is the first time that such a response has been observed on mercury.