Abstract: Microcrystalline 3-(alpha-acetonylbenzyl)-4-hydroxy coumarin (warfarin), having a crystal size not greater than 4.0 microns, is stable for long periods of time, is advantageous in that it is rapidly absorbed from the gastrointestinal tract and thus provides a highly desirable level of bioavailability not previously possible. It is crystallized from a buffered alkaline solution of warfarin by acidification thereof. Compositions thereof are useful for anticoagulant and rodenticide purposes, and methods of making such compositions and of using the new microcrystalline form of warfarin for such purposes are disclosed.