A simple method for the determination of total mercury in biological samples contaminated with inorganic mercury and methylmercury is described. The method is based on the rapid conversion of organomercurials first into inorganic mercury and then into atomic mercury suitable for aspiration through the gas cell of a mercury vapour concentration meter, by a combined tin(II) chloride-cadmium chloride reagent. It was found that if 100 mg of tin(II) chloride alone were added instead of the tin(II) chloride-cadmium chloride reagent, only the release of inorganic mercury influenced the peak deflection of the potentiometer, thus permitting the selective determination of inorganic mercury in the presence of methylmercury. It was possible first to release inorganic mercury then, after re-acidification of the reaction mixture, methylmercury, by adding the tin(II) chloride-cadmium chloride reagent and sodium hydroxide. When total mercury and inorganic mercury were determined separately, the difference between results gave the methylmercury content of the sample.