Two new cancer cell growth inhibiting cyclic peroxides, plakorstatins 1 (1) and 2 (2), were isolated from the Indonesian marine sponge Plakortis nigra. The structures of plakorstatins 1 and 2 including relative configuration were elucidated on the basis of mass and 2D NMR spectroscopic interpretations. These are the first plakortides with an epoxy group in the side chain. Plakorstatin 2 was found to differ from plakorstatin 1 only in the configuration of the epoxide group. Both exhibited moderate cancer cell growth inhibition against the murine P388 lymphocytic leukemia cell line with ED(50) values of 1.1 and 0.91 microg/mL, respectively, for peroxides 1 and 2.