The present invention relates to a single chain binding molecule comprising at least three binding domains, wherein the first binding domain is capable of binding to serum albumin and is positioned at the N-terminus of the second binding domain, said second binding domain is capable of binding to a cell surface molecule on a target cell and the third binding domain is capable of binding to the T cell CD3 receptor complex. Moreover, the invention relates to methods for the production of such binding molecule, a nucleic acid sequence encoding it, a vector comprising said nucleic acid sequence and a host cell expressing the binding molecule of the invention. Furthermore, the invention relates to a pharmaceutical composition comprising a binding molecule of the invention, methods of treatment comprising the step of administering a binding molecule of the invention and the medical use of a binding molecule of the invention.