Digoxin, the most widely used cardiac glycoside, undergoes significant metabolic conversion in many patients to cardioinactive metabolites in which the lactone ring is reduced. This appears to occur within the gastrointestinal tract. An attempt was made to isolate and identify the organisms capable of reducing digoxin from stool cultures obtained from human volunteers. Of hundreds of isolates studied, only Eubacterium lentum, a common anaerobe of the human colonic flora, converted digoxin to reduced derivatives. Such organisms were also isolated in high concentrations from the stools of individuals who did not excrete these metabolites when given digoxin in vivo. When the growth of E. lentum was stimulated by arginine, inactivation of digoxin was inhibited. Neither the presence of these organisms alone nor their concentration within the gut flora appeared to determine whether digoxin would be inactivated by this pathway in vivo.