To develop more effective anti-CD20 reagents for B-cell lymphoma, we designed and constructed a bispecific tetravalent anti-CD20 antibody, 11B8/2F2(ScFvHL)4-Fc, derived from two fully human monoclonal antibodies (mAb), 2F2 and 11B8. 2F2 is a type I CD20 mAb, which is potent in complement-dependent cytotoxicity (CDC) assays but poor at inducing apoptosis, whereas 11B8 is a type II CD20 mAb, which is effective in induction of apoptosis but ineffective in CDC. Our results showed that 11B8/2F2(ScFvHL)4-Fc possessed apoptosis-inducing activity markedly superior to that of 2F2, and even 11B8, 11B8 plus 2F2, and 2F2(ScFvHL)4-Fc, a 2F2-derived monospecific tetravalent antibody developed previously. Interestingly, 11B8/2F2(ScFvHL)4-Fc displayed a similar ability to mediate CDC as 2F2(ScFvHL)4-Fc, although two of its four antigen-binding arms originated from 11B8. To explore why 11B8/2F2(ScFvHL)4-Fc was so potent in both CDC and apoptotic activity, a bispecific divalent antibody composed of 2F2 and 11B8, denoted as 11B8/2F2-ScFvFc, was constructed and characterized. Our results partially explained the reason for the potent CDC and apoptosis-inducing activity of 11B8/2F2(ScFvHL)4-Fc. Further in vivo therapy studies showed that 11B8/2F2 (ScFvHL)4-Fc had a significantly more potent antitumor activity compared with 2F2, 11B8, 2F2 plus 11B8, and 2F2(ScFvHL)4-Fc. These data suggest that 11B8/2F2(ScFvHL)4-Fc may serve as a potential therapeutic agent for B-cell lymphoma. ©2010 AACR.