A series of N6-aminopurine-9-b-D-ribonucleosides and ribose-modified 30-C-methyl analogues substituted at N6-position with a small group like hydroxy, methoxy or amino group or at C2(N6) position have been synthesized and tested against a panel of human leukemia and carcinoma cell lines. N6- Hydrazino-9-b-D-ribofuranosyl-purine (5) displayed the best antiproliferative activity in the low micromolar or submicromolar range against all tested tumor cell lines. The activity of this nucleoside is related in part to ribonucleotide reductase inhibition. C2-modification or 30-C-methylation in N6- substituted adenosine analogues leads to a decrease or loss in activity.