Recombinant Anti-CD28 x Anti-CD3 Bispecific Antibody (F(ab')2-scFv2) is designed to be expressed as two chains combined via a hinge region. The scFv of the anti-CD28 specificity is fused to the C terminus of the anti-CD3 Fab fragment, or vice versa. This format comprises four binding sites (two same scFv regions and two same Fab fragments), two for each antigen. This BsAb can improve T-cell activation. It is designed for the research of Tumors therapy.