A multivalent antibody fusion protein which comprises an immunoglobulin moiety, for example a Fab or Fab&prime； fragment, with a first specificity for an antigen of interest, and further comprises two single domain antibodies (dAb) with specificity for a second antigen of interest which are a VH/VL pair, wherein the two single domain antibodies are linked by a disulfide bond. Also provided are particular dual specificity antibody fusion proteins and other antibody fragments which are stabilised by a disulfide bond.
