Ribonucleic acid in a mixture of crude nucleic acids is selectively hydrolyzed with 5'-phosphodiesterase to 5'-ribonucleotides without hydrolyzing desoxyribonucleic acid in the mixture. Selective hydrolysis is carrying out by contacting the crude nucleic acid mixture with 5'-phosphodiesterase immobilized on a polymer carrier. The crude mixture of nucleic acids is preferably obtained by aqueous extraction of microorganisms that have previously been extracted with ammonia and a lower alcohol to remove lipids. The polymer carrier is preferably a copolymer of glycidylmethacrylate, allylglycidylether, methacrylamide and methylene-bis-methacrylamide.