• Incidence and severity of adverse events through 1 month after completing treatment [ Time Frame: During treatment and through 1 month after completing treatment ] [ Designated as safety issue: Yes ]
  
• Incidence and grade of clinical laboratory abnormalities through 1 month after treatment [ Time Frame: During treatment and through 1 month after completing treatment ] [ Designated as safety issue: Yes ]
  

• Disease response by the International Workshop to Standardize Response Criteria for Non-Hodgkin's Lymphomas 2 weeks after completion of the first treatment cycle [ Time Frame: Two and four weeks after completion of the first and second cycles, respectively ] [ Designated as safety issue: No ]
  
• Immunogenicity by incidence of anti-rIL-21 antibodies up to 1 month after treatment [ Time Frame: During treatment and up to 1 month after completing treatment ] [ Designated as safety issue: Yes ]
  

Drug: recombinant human interleukin 21 and rituximab
rIL-21 (30, 100, 150 ug/kg) IV rituximab (375 mg/m2) IV

This is a Phase 1 open-label dose-escalation study of rituximab + rIL-21 combination therapy, administered once weekly for 4 weeks following an initial treatment with one dose of rituximab alone to patients with B-cell non-Hodgkin's lymphoma (NHL) who have failed prior therapy(ies). A standard dose of rituximab will be used. Increasing doses of rIL-21 will be studied sequentially in different groups of patients, starting with 30 μg/kg.

Before starting treatment with rituximab + rIL-21, patients will be treated with one dose of rituximab alone to look for rituximab infusion-related symptoms (such as fever, chills, and rigors). Patients who have severe infusion-related side effects after the first dose of rituximab will not go on to receive IL-21. Those who do not have unacceptable rituximab-related side effects will receive intravenous rIL-21 at least 60 minutes after completing the rituximab infusion at the rest of the weekly dosing visits. Patients will be evaluated for safety over the course of the study. Disease evaluation (tumor restaging) will be performed 2 weeks following completion of the first 4-week treatment cycle. Patients with stable disease or better at this evaluation may go on to receive a second 4-week treatment cycle of rituximab + rIL-21. Patients may be in the study for 2 to 4 months.