Cytostatic linear and cyclo-depsipeptides, "Dolastatin 16", "Dolastatin 18", and "Dolastatin 17", are disclosed which are obtained from Indian Ocean shelless mollusk Dolabella auricularia. Methods of using the substances to inhibit malignant cell growth associated with neoplastic diseases in animals and humans are also disclosed, as well as pharmaceutical preparations containing Dolastatin 16, Dolastatin 17, and Dolastatin 18.