An open-label, dose-escalation trial of the safety, tolerability, pharmacokinetics (PK), biological, and clinical activity of DI-Leu16-IL2 administered to patients with CD20 (B-lymphocyte antigen CD20) positive Non-Hodgkin Lymphoma (NHL) that have failed standard rituximab-containing therapy. Following peripheral blood B cell depletion with rituximab (if needed) each patient will receive DI-Leu16-IL2 administered as a subcutaneous (SC) injection for three consecutive days every three weeks.