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The marine natural product, didemnin B, is a 7-amino acid, cyclic depsipeptide that inhibits G1 cell cycle progression at nanomolar concentrations by undefined mechanisms. It has been reported to exhibit immunosuppressive activities in animals and is undergoing clinical trials as a potential antineoplastic drug. In addition, at higher concentrations, didemnin B has been shown to inhibit in vivo and in vitro protein synthesis. However, the mechanisms by which inhibition is achieved are unknown. To investigate didemnin's various modes of action, an affinity column was synthesized and used to purify didemnin-binding proteins. The major retained protein was the 49-kDa guanine nucleotide-binding elongation factor, EF-1 alpha, which was identified by peptide sequence analysis. Moreover, didemnin binds EF-1 alpha only in the presence of GTP but does not inhibit the GTPase activity of EF-1 alpha. Therefore, EF-1 alpha is likely to be the intracellular target responsible for didemnin B's ability to inhibit protein synthesis. Furthermore, this specificity of didemnin affinity for the GTP-bound conformation of a guanine nucleotide-binding protein with homology to the Ras superfamily suggests a possible mode of action for didemnin's antiproliferative activity.