Patients receive tirapazamine by 2 hour intravenous infusion (hours 0-2) followed 2 hours later by a 30 minute intravenous infusion of cyclophosphamide. This course is repeated every 3 weeks in patients with partial/complete response or stable disease for a maximum of 1 year. Cohorts of 3-6 patients each are treated at each dose level of tirapazamine. Dose escalation of tirapazamine occurs when 0 of 3 patients or 1 of 6 patients has experienced dose limiting toxicity (DLT). If DLT is experienced in 1 of 3 patients at a given dose level, up to 3 additional patients are treated at that same dose level. If none of the 3 additional patients at that dose level experiences DLT, the dose is escalated. If DLT is experienced in 1 or more of the additional 3 patients, the maximum tolerated dose (MTD) has been exceeded and 3 patients are treated at the next lower dose level (defined as the MTD). A total of six patients are treated at the MTD. If DLT is proved to be neutropenia, patients must then also meet the additional eligibility criteria listed for stratum 2. If neutropenia continues to be the DLT in stratum 2, then additional patients receive subcutaneous filgrastim (granulocyte colony-stimulating factor; G-CSF) beginning 24 hours after cyclophosphamide. A second MTD may be determined for chemotherapy with G-CSF. Patients are followed every 6 months for 4 years, and then annually thereafter.