Didemnin B is a member of a class of compounds, derived from a marine source, undergoing phase II study. Twenty-two patients with relapsed myeloma were treated with didemnin B at an initial dose of 4.9 mg/m2, given once every 28 days. All were evaluable for toxicity, and 15 were evaluable for myeloma response. No tumor regressions occurred in the 15 patients evaluable for response. Vomiting was the major toxicity, occurring in 73% of patients despite vigorous pre- and post-treatment medication with at least three intravenous antiemetics. Two instances of grade 4 hypersensitivity reaction occurred. We conclude that didemnin B has no activity at this dose and schedule in myeloma that has relapsed after one or two prior therapeutic regimens.