Tryptophan mustard, 0.3 mg/kg i.v. once weekly, was given to 38 patients with plasmacytic myeloma. Of these, 27 were treated for 30 days or more and therefore were considered to be eligible for evaluation of response. Improvement in one or more parameters occurred in 14 patients. Six patients received benefit in two parameters or more. Toxicity consisted of leukopenia and thrombocytopenia but this did not cause serious complications. Nausea and vomiting occurred in 18 patients and necessitated cessation of therapy in 4. Tryptophan mustard appears to possess activity against plasmacytic myeloma and may be useful when other therapy has failed.