8-Chloroadenosine, an active dephosphorylated metabolite of the antineoplastic agent 8-chloroadenosine 3 0 ,5 0 -monophosphate (8-Cl-cAMP), induces growth inhibition in multiple carcinomas. Here we report that 8chloroadenosine inhibits growth in human promyelocytic leukemia HL-60 cells by a G 0 /G 1 phase arrest and terminates cell differentiation along the granulocytic lineage. The mechanism of 8-chloroadenosine-induced G 0 /G 1 arrest is independent of apoptosis. The expressions of cyclin D1 and c-myc in HL-60 are suppressed by 8-chloroadenosine, whereas the cyclin-dependent kinases inhibitor p21 WAF1/CIP1 is up-regulated. 8-Chloroadenosine has less effect on the expressions of cyclin-dependent kinase (cdk)2 and cdk4, G 1 phase cyclin-dependent kinases, and only moderately induces the expression of transforming growth factor b1 (TGFb1) and the mitotic inhibitor p27 KIP1 . Telomerase activity is reduced in extracts of 8-chloroadenosine treated HL-60 cells, but 8-chloroadenosine does not directly inhibit the catalytic activity of telomerase in vitro. Therefore, anti-proliferation of HL-60 cells by 8-chloroadenosine involves coordination of cyclin D1 suppression, reduction of telomerase activity, and up-regulation of p21 WAF1/CIP1 that arrest cell-cycle progression at G 0 /G 1 phase and terminate cell differentiation.