Important recent advances have been made in our understanding of antigen processing of cytoplasmic antigens and presentation by class I molecules of the MHC. Peptide transporter-like molecules encoded within the MHC have been characterized and have, by transfection, corrected some of the presentation-mutant cell lines. The nature of peptide-MHC class I interactions has been clarified by further resolution of the HLA A2 and B27 crystals and elution of peptides. The differences between antigenicity and immunogenicity for viral antigens have been highlighted by studies in transgenic animals.