The microbial synthesis of some purine 2'-amino-2'-deoxyribonucleosides from purine bases and 2'-amino-2'-deoxyuridine is described. Various bacteria, especially Erwinia herbicola, Salmonella schottmuelleri, Enterobacter aerogenes and Escherichia coli, were able to transfer the aminoribosyl moiety of 2'-amino-2'-deoxyuridine to purine bases (transaminoribosylation) in the presence of inorganic phosphate. The optimum conditions for the reaction were pH 7.0 and 63°C. No reaction was observed in the absence of inorganic phosphate and the optimum concentration of it was around 30mM. Adenine, guanine, 2-chlorohypoxanthine and hypoxanthine were transformed to the corresponding 2'-amino-2'-deoxyribonucleosides by the catalytic activity of the wet cell paste of Enterobacter aerogenes AJ 11125. The enzymatically synthesized purine 2'-amino-2'-deoxyribonucleosides were isolated and identified by physicochemical means. 2'-Amino-2'-deoxyadenosine strongly inhibited the growth of Hela cells in tissue culture, and the ED50 was 2.5 μg/ml.