A recombinant single-chain polypeptide engineered to exhibit the full binding and activity of an anti-CD37 monoclonal antibody with potential immunostimulatory and antineoplastic activities. Otlertuzumab binds to CD37 on B-cells, which may result in antibody-dependent cell-mediated cytotoxicity (ADCC) and apoptosis. CD37 is a transmembrane glycoprotein expressed at high-levels on B cells and to a lesser extent on T cells and myeloid cells. This agent may have a longer half-life in vivo than conventional monoclonal antibodies.