OUTLINE: This is a dose-escalation, multicenter study. Patients receive PS-341 IV on days 1 and 4. Treatment repeats every 14 days for at least 2 courses in the absence of unacceptable toxicity or disease progression. Patients with stable or responding disease may receive additional courses at the discretion of the treating physician. Cohorts of 3-6 patients receive escalating doses of PS-341 until the maximum tolerated dose (MTD) is determined. The MTD is defined as the dose at which 2 of 3 or 6 patients experience dose-limiting toxicity. Patients are followed at 4 weeks.