A rare type of antibody that spontaneously binds to each self (homodimerizes) has been described. This self-binding (autophilic) antibody provides stronger protection against bacterial infection than a non self-binding antibody with identical specificity and affinity due to increase of polymeric avidity. A peptide derived from the self-binding domain of the autophilic antibody was crosslinked to the Fc carbohydrate of two monoclonal antibodies specific for the B-cell receptor (BCR) of a murine and a human B-cell tumor. Peptide-crosslinked antibodies bind to themselves on solid phase ELISA as homodimer and establish in solution a monomer-dimer equilibrium. Autophilic antibodies bind to their respective tumor target cells with increased efficiency as determined by FACS analysis. They also induce twice the amount of apoptosis of target tumor cells than the control antibodies. Furthermore, the modified antibodies inhibit tumor growth in culture more efficiently than the control antibodies. Criss-cross protocols in FACS, apoptosis, and growth inhibition indicate the specificity of targeting the BCR with autophilic antitumor antibodies. The chemical approach of increasing the binding of antibodies without creating chemically crosslinked dimers mimics naturally occurring autophilic antibodies and represents a simple and attractive alternative to chemical dimerizing and antibody engineering techniques for improving their antitumor effect. Furthermore, these results provide a guide to incorporate the self-binding peptide into the structure of antibodies using modeling and molecular crafting techniques.