The didemnin class of biologically active cyclodepsipeptides, isolated from the marine tunicate Trididemnun solidum, has shown considerable antitumor, antiviral, and immunosuppressive activities. Didemnin B (DB) and most other natural didemnins contain a common macrocycle and differ only in the composition of the side chain. In the present invention, structural modifications were introduced in the side chain to afford several didemnin analogues for structure-activity relationship studies. The latter have shown that the linear side chain portion of the didemnin core structure can be altered and, in some cases, provide significant gains in bioactivities. Of the compounds synthesized the new [Ala.sup.9 ] didemnin B is the most active against the L1210 (murine leukemia) cell line, at the 0.04 ng/mL level.