The two new marine sponge (Phakellia sp., western Pacific Ocean) constituents, phakellistatin 10 [1] and 11 [2], were found to be cyclic octapeptides that significantly inhibited growth of the murine P-388 lymphocytic leukemia (ED50 values of 2.1 and 0.20 micrograms/ml, respectively) and human cancer cell lines. The structures were established based on results of extensive tandem ms/ms and high-field (500-MHz) 2D 1H- and 13C-nmr analyses. All of the amino acid units (except Trp, not determined) were found to correspond to the (S)-configuration.