The extract of an Indonesian marine sponge Haliclona sp. showed potent cytotoxicity against human solid cancer cell lines, MCF-7 (breast), LNCap (prostate), Caco-2 (colon) and HCT-15 (colon) cells. Study on nuclear morphological changes and flow cytometric analysis suggested that the component(s) in the extract would induce an apoptosis to these cancer cells. Bioassay-guided isolation yielded two pentacyclic alkaloids, papuamine (1) and haliclonadiamine (2), which inhibited cell proliferation of six human cancer cell lines with IC50 values of 0.93-1.50 and 1.00-4.44 µM, respectively. Compounds 1 and 2 accumulated lymphoma U937 cells at sub-G1 phase and induced a condensation of chromatin and fragmentation of nucleus.