The present invention relates to compositions comprising polypeptides, especially polypeptides capable of specifically binding predetermined antigens. The polypeptide in the composition comprises at least two antigen binding sites. These at least two antigen binding sites are located on a single polypeptide chain. One of the at least two antigen binding sites specifically binds the human CD3 antigen. The polypeptide may exist in both monomeric form and multimeric form. The multimeric form of the polypeptide constitutes no more than 5% of the total weight of the combined monomeric and multimeric forms of said polypeptide.
