Recombinant Anti-CD28 x Anti-CD3 Bispecific Antibody (Fab-Fv) is designed to be expressed as two polypeptide chains. The heavy chain Fd fragment and light chain of an anti-CD28 antibody are fused with the VL and VH domains of an anti-CD3 antibody respectively. It can form natural in vivo heterodimerization. This BsAb can improve T-cell activation. It is designed for the research of Tumors therapy.