Bispecific antibodies (BsAb) are promising therapeutic tools in tomorrow's medicine. Expression systems favoring efficient heterodimerization of intermediate-sized bispecific antibodies will significantly improve existing production methods. By C-terminal fusion of scFv molecules to the Fd- and the L-chains efficient heterodimerization in mammalian cells was obtained and a novel intermediate sized, disulfide stabilized BsAb could be efficiently produced. This type of antibody derivative easily allows for the production of trispecific antibodies, BsAb with bivalent binding for one antigen, or immunoconjugates.