The major histocompatibility complex (MHC) class I molecule is responsible for presenting peptide antigens at the cell surface for recognition by cytotoxic T lymphocytes. Several chaperone molecules interact with the MHC class I heavy chain and release when the MHC groove folds around peptide. Two additional proteins, invariant chain and amyloid precursor-like protein 2 (APLP2), interact specifically and stably with MHC class I molecules that have folded peptide-binding grooves. Invariant chain and APLP2 also affect MHC class I cell-surface expression, and so may play a part in MHC class I trafficking. Association of APLP2 with the MHC class I molecule appears to be regulated by a viral protein, the adenovirus E3/19K protein. Analysis of the interactions of these proteins with each other and with MHC class I will clarify how presentation of antigens by MHC class I is controlled by events that occur subsequent to MHC class I folding.