Purpose In search for new drugs derived from natural products for the possible treatment of cancer, we studied the action of agelasine B, a compound purified from a marine sponge Agelas clathrodes. Methods Agelasine B was purified from a marine sponge Agelas clathrodes and assayed for cytotoxicity by MTT on two human breast cancer cells (MCF-7 and SKBr3), on a prostate cancer cells (PC-3) and on human fibroblasts. Changes in the intracellular Ca2? concentrations were asses- sed with FURA2 and by confocal microscopy.Determination of Ca2?-ATPase activity was followed by Pi measurements. Changes in the mitochondria electrochemical potential was followed with Rhodamine 123. Apoptosis and DNA frag- mentation were determined by TUNEL experiments. Results Upon agelasine B treatment, cell viability of both human breast cancer cell lines was one order of magnitude lower as compared with fibroblasts (IC50 for MCF- 7 = 2.99 lM; SKBr3: IC50 = 3.22 lM vs. fibroblasts: IC50 = 32.91 lM), while the IC50 for PC-3 IC50 = 6.86 lM. Agelasine B induced a large increase in the intra- cellularCa2?concentration inMCF-7, SKBr3, andPC-3 cells. By the use of confocal microscopy coupled to a perfusion system, we could observe that this toxin releases Ca2? from the endoplasmic reticulum (ER). We also demonstrated that agelasine B produces a potent inhibition of the ER Ca2?- ATPase (SERCA), and that this compound induced the frag- mentation of DNA. Accordingly, agelasine B reduced the expression of the anti-apoptotic protein Bcl-2 and was able to activate caspase 8, without affecting the activity of caspase 7. Conclusions Agelasine B in MCF-7 cells induce the activation of apoptosis in response to a sustained increase in the [Ca2?]i after blocking the SERCA activity. The reproduction of the effects of agelasine B on cell viability and on the [Ca2?]I obtained on SKBr3 and PC-3 cancer cells strongly suggests the generality of the mechanism of action of this toxin.