The goal of this clinical research study is to find out if the study drug, ONTAK (denileukin diftitox), can shrink or slow the growth of B-cell non-Hodgkin's lymphoma (NHL) in patients whose disease has not responded to prior treatments, or has relapsed after an initial response to prior treatments. The safety of treatment with ONTAK will also be studied. The hypothesis is that patients with relapsed or refractory B-cell NHL and mild to moderate myelosuppression treated with ONTAK at a new dosing regimen will respond sufficiently to warrant further study.