The new cancer cell growth (human and murine) inhibitory (GI,, 0.3 to 3.3 pg/ml) cycle-octapeptide axinastatin 5 (2) was isolated in 3.8 x lo-'% yield from the Western Indian Ocean marine sponge Axinella cf. carteri. Structural elucidation was achieved by high field (400 and 500 Hliz) 2 D-NHR techniques and the amino acid sequence was confirmed by results of NHR and high resolution mass spectral MS/MS interpretations. Other peptide cancer cell growth inhibitory constituents of this orange sponge were found to be the cyclic hepta- and octapeptides axinastatin 1 and hymenistatin 1 (1).