The R. L. McGregor Herbarium houses approximately 400,000 vascular plant specimens. The majority of these comprise exsiccatae, but the collection also includes seed, boxed, and fluid-preserved specimens. The collection is focused on the Central Grassland region of North America, with specimens from the Great Plains comprising approximately two-thirds of the collection. Families with especially strong taxonomic and geographic representation include Asteraceae, Cyperaceae, Fabaceae, Juncaceae, Lemnaceae, Plantaginaceae, and Poaceae.