The Republic of Palau marine sponge Axinella sp. was found to be an exceptionally productive source of cell growth inhibitory substances. The strongly antineoplastic polyether macrocyclic lactones halichondrin B (1) and homohalichondrin B (2) were isolated in 1.2 x 10(-6)% and 5.4 x 10(-7)% yields, respectively. In addition to axinastatin 1 (3), two new and cytostatic (GI50 values of 0.35 to 0.0072 microgram/mL against six human cancer cell lines) cycloheptapeptides designated axinastatins 2 (4) and 3 (5) were discovered in 1.4 x 10(-6)% and 1.25 x 10(-6)% yields. Structures were elucidated by high-resolution FABMS and tandem MS/MS techniques augmented by high-field (400 and 500 MHz) 2D-NMR spectral analyses. The absolute configurations were established by a combination of hydrolysis, derivatization, and chiral gas chromatographic methods.