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.