Peptides that are presented to T-cells by class I major histocompatibility complex molecules are derived from cytosolic proteins. They are generated in the cytosol and translocated into the endoplasmic reticulum (ER) by the transporters associated with antigen processing (TAP molecules). Competition experiments suggest that TAP molecules can specifically translocate a wide range of peptides from 8-13 amino acids long; longer peptides are less likely to be transported. A photoactivatable peptide derivative has been used to demonstrate that competition for transport into the ER reflects competition for a specific peptide-binding site on the TAP molecule. Class I molecules bind the translocated peptides in the ER thereby allowing their transport to the cell surface. The assembly of the class I-peptide complex in the ER is tightly regulated. The evidence suggests that class I heavy chains first dimerize with beta 2-microglobulin in a process mediated by the chaperone calnexin. The class I-beta 2-microglobulin dimer then physically associates with TAP molecules and is released for transport when it binds a peptide.