High resolution (2A) X-ray diffraction data have been measured for J539. This will greatly extend the structure analysis of this Galactan-building Fab. High resolution X-ray data have been measured for the Fabs of two monoclonal antibodies to lysozyme complexed to lysozyme. A molecular replacement analysis of these structures is in progress. Some structural factors relating to antibody assembly have been analyzed. In particular, the nature of the interaction between CH1 and CL has been studied. A proposed model for the structure of the IgE Fc has been constructed.