A multivalent antibody fusion protein which comprises an immunoglobulin moiety, for example a Fab or Fab' 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, wherein the two single domain antibodies are linked by a disulfide bond. There is also provided particular dual specificity antibody fusion proteins comprising a Fab or Fab' fragment and one or more single domain antibodies which may be stabilised by a disulfide bond therebetween.