Summary: An electron-probe microanalysis of tetrahedrite occurring together with the latest-deposited pyrite-cinnabar association in the Chiprovtsi lead-silver deposit yielded more than 16 wt. % Ag and more than 18 wt. % Hg. X-ray powder examination established an extremely large unit-cell edge, a 10·64±0·01 Å. Vickers microhardness 223 to 237 kg/mm². The chemical formula derived on the basis of the analyses made is quite near to Cu₆⁺(Cu⁺, Ag⁺)₄(Hg,Zn)₂(Sb,As)₄S₁₁₃. There is not any antiparallelism between Ag and Hg in tetrahedrite. The name proposed for all stages of simultancous isomorphic replacement along the Cu⁺-Ag⁺ and Cu²⁺-Hg²⁺(Zn²⁺,Fe²⁺) directions is argentian mercurian tetrahedrite. The upper limits of the Ag and Hg content are expected to be Cu₆Ag₄Hg₂(Sb,As)₄S₁₃. It is the six copper atoms (Cu4+·Cu22+), sited in the Laves' polyhedra, that are replaced by Ag⁺ and Hg²⁺ respectively.