ImuVert, a new biological response modifier of bacterial origin, was evaluated in a Phase II trial of patients with metastatic prostate cancer. Sixteen patients with hormone refractory measurable or evaluable disease were treated with ImuVert 1.0 mg subcutaneously once weekly for 5 weeks. After a 1 week rest period, the dose was escalated to 3.0 mg weekly for 5 additional weeks. Eleven patients received the full 10 weeks of therapy. Toxicity consisted of mild transient flu-like symptoms as well as the development of tenderness and induration at injection sites. No patient responded to treatment. ImuVert at this dose and schedule is inactive in advanced prostate cancer.