Although most microorganisms with genetic blocks in the purine nucleotide sequence excrete breakdown products, a coryneform bacterium was found to accumulate intact 5'-nucleotides in the extracellular medium. Adenineless mutants accumulated 0.4 to 0.6 g of inosine-5'-monophosphate per liter of broth. The yield of this nucleotide was increased to 0.8 to 0.9 g per liter when such mutants were mutated to xanthine dependence. Induction of a specific guanine requirement in adenineless auxotrophs resulted in cultures capable of producing high yields of xanthosine-5'-monophosphate (3 to 4 g per liter). Pure xanthosine-5'-monophosphate was isolated from broth by a procedure involving ion-exchange chromatography, charcoal adsorption, and barium precipitation.