Mammalian prion protein (PrP) is a normal cellular protein (PrPc) which through post-translational modification produces the infectious prion protein (PrPsc). We have shown, using mass spectrometry, that synthetic peptides containing three or four copies of an octapeptide repeat sequence (PHGGGWGQ), found in a highly conserved N-terminal domain of PrP, preferentially bind copper over other metals. Peptides from the analogous region of chicken PrP, which contains an N-terminal repeat domain of the hexapeptide (NPGYPH), showed similar specificity for copper binding. In addition, gel filtration chromatography demonstrated concentration dependent binding of copper to the mammalian tetra repeat PrP peptide. These results suggest that PrP may be a copper binding protein in vivo.