Michał Eustachy Brensztejn compiled the ‘Archaeological Inventory of the Kovno Gubernia’ in 1907. The manuscript was not published, and only in 2010 was it discovered in the archives of the State Archaeological Museum in Warsaw. The Lithuanian Institute of History and the State Archaeological Museum in Warsaw initiated a project to publish the ‘Inventory’ as the third part of the Ostbalticum project. This paper gives some preliminary insights and a short description of the manuscript as a source for Lithuanian archaeology. It analyses the sources used by Brensztejn, describes the process of identification of place-names, discusses the reliability of the records and the novelty of these data, and shows some characteristic mistakes that the author of the ‘Inventory’ made. A puzzle of artefact collection from Jagminai is presented as a brief case study. Thanks to the oral tradition recorded by Brensztejn, the identification of the site was possible.