A bioassay-directed investigation of the cytotoxic constituents of the Japanese sea hare Dolabella auricularia resulted in the isolation of two cytotoxic compounds designated dolastatin H (2) and isodolastatin H (3) from the wet animals at the yields of 9 x 10(-7)%. On the basis of spectroscopic analysis, these compounds were shown to be new peptides that were closely related to dolastatin 10 (1) isolated from Western Indian Ocean specimens of this animal. The notable structural feature of these new compounds, 2 and 3, is that 3-phenylpropane-1,2-diol is attached through the ester linkage to the C-terminus of a tetrapeptide containing unusual amino acids. The absolute stereostructures of 2 and 3 were unambiguously determined by enantioselective total synthesis. A cytotoxicity test for synthetic 2, 3, and their C-2 epimers revealed that the stereochemistry of the 3-phenylpropane-1,2-diol moiety on the C-terminus plays an important role in their cytotoxicity. In vivo antitumor activity against murine P388 leukemia was evaluated, and it was shown that isodolastatin H (3) exhibited antitumor activity a little weaker than that of dolastatin 10 (1).