X-ray structures of three different membrane proteins in complex with antibody fragments have been published. The binding of Fv or Fab fragments enlarges the hydrophilic part of integral membrane proteins, thereby providing additional surface for crystal contacts and space for the detergent micelle. In all reported cases, antibody binding was either essential for the crystallisation of the membrane protein or it substantially improved the diffraction quality of the crystals. Antibody-fragment-mediated crystallisation appears to be a valuable tool in particular for membrane proteins with very small hydrophilic or flexible domains.