The history of the German cooperative banks has enjoyed renewed attention in recent years. This applies specifically to the current central cooperative bank, DZ BANK, whose origin lies in the Prussian Central Cooperative Bank ("Preußenkasse") founded in 1895. What previous research has in common is that it did not draw on the major, long-forgotten, 171-meter long "I. HA. Rep. 177, Preußische Zentralgenossenschaftskasse" archive collection in the Secret State Archives of the Prussian Cultural Heritage Foundation ("Geheimes Staatsarchiv Preußischer Kulturbesitz"). The present paper is the result of a first viewing of these mostly-intact, archival records, which open new approaches for historians researching the cooperative movement during the period from the 1890s to about 1928. The emphasis of the extant documents lies on the middle tier of the three-level structure of the credit unions, with the central banks and regions which have hardly been investigated in previous research due to the lack of meaningful sources. The classification of the majority of the records by central bank permits not only a focus on regional history, the archives as a whole are also a rich primary source for the study of the relationship between the central banks and the "Preußenkasse". The same applies to the topic of agricultural lending, where the surviving documents range from critical individual cases to analyses of different geographical areas, and on to the implementation of numerous lending programs. When and if researchers will gain access to the files in the collection through suitable finding aids is currently not yet foreseeable however.