Digitonin is a steroidal saponin extracted from foxglove (Digitalis purpurea) seeds that binds cholesterol and permeabilizes cell membranes. Unlike digitoxin, the principal cardiac glycoside of foxglove, it has no appreciable effect on the heart. Analytical chemists use digitonin as a reagent to precipitate and quantify free cholesterol in foods and biological samples. Its membrane-disrupting action also makes it a tool in cell biology for releasing intracellular contents without destroying organelles. Because it permeabilizes membranes selectively, it is dosed carefully to avoid destroying the very structures under study.