OUTLINE: This is a dose-escalation, multicenter study. Patients receive PS-341 IV over 30 minutes on days 1, 4, 8, 11, 15, 18, 22, and 25 followed by a 2-week rest. Treatment repeats every 6 weeks in the absence of disease progression or unacceptable toxicity. 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 6 patients experience dose-limiting toxicity.