In a study of the relationship of magnetic anomalies to the magnetite content and structure of layered Precambrian Fe-formations, the effective induced and remanent magnetizations of the Biwabik Iron-Formation were determined by analyses of aeromagnetic anomalies recorded 1000 ft above the Biwabik in areas of known geology. The induced magnetization along the layers is relatively insignificant because the formation is almost perpendicular to the earth's magnetic field. For the relatively unmetamorphosed Fe-formation of the Main Mesabi district, the dominant magnetization was found to be across the layers, about 0.012 gauss, and most likely induced. For the strongly metamorphosed Fe-formation of the East Mesabi district, the dominant magnetization was found to be along the layers, about 0.100 gauss, and remanent. These values are used as bases for explaining aeromagnetic anomalies from correlatives of the Biwabik Fe-formation in the Gunflint, Cuyuna, and Gogebic districts.