We previously isolated aaptamine, a benzonaphthyridine alkaloid, from marine sponge Aaptos suberitoids. In this study, we investigated the anti-proliferative effect of aaptamine on chronic myeloid leukemia (CML) K562 cells. Aaptamine inhibited growth of K562 with a GI50 as 10 mu M, and arrested cell cycle at G2/M phase. Western blot analysis indicated that aaptamine induced p21 expression in K562 cells. Moreover, p21 promoter was activated by aaptamine treatment in p21 transfected K562 cells. Since K562 is p53 negative, aaptamine was demonstrated to be a p53-independent p21 inducer in CML cells.