The invention provides a method for predicting whether a binding peptide, which binds to a target peptide presented by a Major Histocompatibility Complex (MHC) and is for administration to a subject, has the potential to cross react with another peptide in the subject in vivo. The method comprises the steps of identifying at least one binding motif in the target peptide to which the binding peptide binds; and searching for peptides that are present in the subject that comprise the at least one binding motif and that are not the target peptide. The presence of one or more such peptides indicates that the binding peptide has the potential to cross react in vivo.