The in vitro induction of apoptosis by N6-substituted derivatives of adenosine and adenine was investigated in HL-60 cells. Using reversed phase HPLC/MS analysis we demonstrated that both N6-substituted derivatives of adenosine and adenine are phosphorylated within cells to the monophosphate level. While N6-substituted derivatives of adenosine were phosphorylated by adenosine kinase and corresponding mononucleotides were produced in large quantities, N6-substituted derivatives of adenine were converted into the corresponding mononucleotides via the phosphoribosyl transferase pathway, which yielded 50-100 times lower amounts of the mononucleotides than the adenosine kinase pathway. Accordingly, N6-substituted derivatives of adenine were relatively inefficient inductors of apoptosis at the concentrations applied. Inhibitors of adenosine kinase that abrogated the formation of monophosphates from N6-substituted derivatives of adenosine completely prevented cells from going into apoptosis. These results consistently support the idea that pro-apoptotic effects of N6-substituted derivatives of adenosine are related to their intracellular conversion into corresponding mononucleotides which eventually trigger apoptosis when accumulated beyond certain level. Intracellular accumulation of mononucleotides derived from the corresponding N6-substituted derivatives of adenosine led to a rapid decrease in ATP production and consequently to apoptosis induction. Nevertheless, the detailed mechanism is unknown and must be further elucidated. Apoptosis, induced by N6-substituted derivatives of adenosine, was accompanied by a distinct caspase-3 activation. However, a broad spectrum caspase inhibitor, z-VAD-fmk, failed to prevent cells from death, thereby indicating that caspases alone were not mediators of cell death.