The present invention provides recombinant antibody fragments which include a variable domain which has been modified by the addition of a tail sequence to its C-terminal end. The tail sequence comprises a terminal cysteine residue and an amino acid spacer and does not substantially affect the fragment's target-binding affinity. The present invention also provides pharmaceutical compositions comprising the described antibody fragments and a pharmaceutically acceptable carrier and methods of delivering an agent to cells of interest in a subject using the fragments as delivery vehicles. The invention further provides compositions comprising the described antibody fragments for the in vitro detection and measurement of target molecules which bind to the fragments and method of determining the presence or amount of such targets in a biological sample by contacting the sample with such compositions.