On the basis of unpublished archive materials (see G.M. Bongard-Levin, V.A. Gaibov, G.A. Koshelenko. A History of Excavations at Dura Europos // Parfyansky Vystrel [Parthian Shot]. Ed. by G.M. Bongard-Levin and Yu.N. Litvinenko. M., 2003) the contributors make a detailed description of the excavations of the Mithraeum at Dura Europos in 1933/34, of the work over the Preliminary Report published in 1939 and over the Final Report, which was never pub- lished. The core of the article is the analysis of F. Cumont's conception (supported by M.I. Ros- tovtzeff) of the spread of Mithraism in the Roman Empire (as it was reflected in the analysis of the Mithraeum at Dura Europos). The authors show that the new materials prove this conception to be true: the most important role in forming the iconography of the Mithraist cult (as well as the system of beliefs and rituals) belonged to the population of East Anatolia of late Hellenistic and early Roman times.