Anaerobic growth of Staph. aureus upon a mixed amino-acid medium containing pyruvic acid and other necessary nutrients (Fildes et at., Brit. J. Exp. Path. 17, 1936) was only possible, under the conditions of test, upon addition of uracil (2:6-dihydroxypyrimidine). Of 21 related compounds tested, none had a comparable effect in permitting growth. The significance of this to the distribution and function of uracil in nature is briefly discussed.