Abstract: A method of isolating and identifying peptide substrates for a protein kinase is disclosed. The method involves a combination of size exclusion and gallium-based metal affinity chromatography. The method includes the steps of incubating a protein kinase with a peptide library in the presence of kinase reaction components, the library comprising library members; separating library members from the kinase reaction components using size exclusion chromatography to give a pool of phosphopeptides and unphosphorylated peptides; contacting the pool with immobilized gallium ions to form chelated phosphopeptides; eluting chelated phosphopeptides away from the gallium ions to give eluted phosphopeptides; sequencing the eluted phosphopeptides, whereby a preferred amino acid sequence of a preferred peptide substrate for a protein kinase is elucidated.