A phase I/II study of veltuzumab combined with milatuzumab in relapsed and refractory non-Hodgkin's lymphoma. Both agents are well-tolerated in early phase clinical testing with infusion reactions as the primary observed toxicity. Preclinical testing in vitro and in vivo have demonstrated single agent activity for both veltuzumab and milatuzumab. In mantle cell lymphoma cell lines and SCID mouse models, synergist effects were observed when milatuzumab was combined with rituximab. Veltuzumab has several advantages over rituximab including slower off-rates, shorter infusion times, higher potency, and improved therapeutic responses in animal models. Previous and ongoing clinical investigations support the concept of combining monoclonal antibodies in NHL.