A pilot experiment carried out on three pigs have confirmed that interaction between inorganic mercury (203HgCl2) and selenium (Na275SeO3) after single intraperitoneal injections are qualitatively uniform in mice and pigs. The detoxifying effect of selenium on mercury toxicity seems to be due to a formation of a biologically inactive complex containing the elements in an equimolar ratio. The complex is unable to pass biological barriers, placenta and choroid plexus and is stored in the liver and the spleen.