X-ray diffraction data have been collected from crystals of the Fab of CC49, a murine monoclonal antibody against solid adenocarcinoma, and molecular replacement analysis of the crystal structure is in progress. The known structures of Fabs have been analyzed to determine which framework residues need to be preserved in the 'humanization' of xenogeneic antibodies by CDR-grafting. A model of the extracellular portion of the alpha-subunit of the human high-affinity IgE receptor has been built. The binding of various viral and self peptides to the murine class I MHC antigen, H-2Ld, has been modelled.