Didemnin compounds were first reported by Rinehart et al. as isolates from the Caribbean tunicate Trididemnum solidum in 1981. These cyclic depsipeptides possess a variety of biological activities including in vitro and in vivo antiviral, antitumor, and immunosuppressive activities. They are potent inhibitors of L1210 leukemia cells in vitro, and are also active in vivo against P388 leukemia and B16 melanoma. Didemnin B, a particularly active compound of this class, is approximately twenty times more cytotoxic than didemnin A in vitro and has undergone phase II clinical trials for antitumor activity. Both didemnins A and B exhibit antiviral activity against DNA and RNA viruses, with didemnin B being more active. The structures of didemnins A and B have been established as 1 and 2, respectively. ##STR1##