The electron transfer reactions between a lipid-modified, pyrolytic graphite (PG) electrode and tetraheme cytochromes c3 have been studied at different ionic strengths by cyclic and square-wave voltammetry. In 0.1 M phosphate buffer (pH 7.6), no signal was detected at the PG electrode coated with a mixed layer of phosphatidylcholine (PC) and cholesterol (CH) (containing 5 mg ml-1 PC). The signals corresponding to the reduction-re-oxidation of cytochrome c3 from Desulfovibrio vulgaris Hildenborough (DvH) were restored by doping the lipid layer with laurate anions. At low ionic strengths (10 mM phosphate), using the same (PC - CH + LA)-modified PG electrode, DvH cytochrome c3 molecules are incorporated into the lipid layer. The stability of the film has been studied after transfer into an electrolyte solution. No incorporation was observed in the case of cytochrome c3 from Desulfovibrio desulfuricans Norway. The results support the electrostatic nature of the interactions between cytochrome c3 and the lipid layer. This approach to the electrochemistry of cytochrome c3 opens new ways for the study of proteinlipid interfaces.