The conditions of synthesis of 9-beta-D-arabinofuranosyladenine, an antiviral nucleoside by intact cells of Escherichia coli capable of adenine transarabinosylation were studied. The cells were grown in meat-peptone broth supplemented with yeast extract. Cytosine arabinoside served as a donor of the arabinose residues. The pH value, temperature of the reaction medium and concentration of phosphate ions in it were shown to be significant factors defining the cell activity. Under optimal reaction conditions: 0.03 M potassium phosphate buffer, pH 6.75; 0.03 M cytosine arabinoside; 0.01 M adenine; 5 per cent of the cells; incubation time 12 hours; incubation temperature 60 degrees C the efficiency of 9-beta-D-arabinofuranosyladenine synthesis reached 90-95 mol % with respect to the initial adenine level.