Previous experiments have shown that short peptides coresponding to naturally processed epitopes of viral antigens can induce a conformational change in the class I heavy chain (HC) to which they bind in the fully assembled molecule. Here, we present evidence that the mechanism for this conformational change may involve binding of peptide to a partially unfolded form of free HC, followed by its subsequent folding. These results may be important for understanding the way in which class I molecules are assembled in vivo, and how certain epitopes are selected for presentation to T cells.