The optical enantiomers of (&)-C-Ado could efficiently and conveniently be separated after a selective enzymatic degradation of (-)-C-AMP to (-)-C-Ado in a reaction mixture containing (A)-C-AMP and 5'-ribonucleotide phosphohydrolase. (+)-C-Ado could subsequently be obtained upon treatment of the remaining (+)-C-AMP with alkaline phosphatase. Of the two enantiomers, only the (-) form showed significant cytostatic and antiviral activity. The (+) enantiomer was totally inactive.