We introduce a versatile fluorescent peptide reporter of protein kinase activity. The probe can be modified to target a desired kinase by changing the kinase recognition motif in the peptide sequence. The reporter motif contains the Sox amino acid, which generates a fluorescence signal when bound to Mg2+ present in the reaction mixture. The phosphorylated peptide exhibits a much greater affinity for Mg2+ than its unphosphorylated analogue and, thus, a greater fluorescence intensity. Product formation during phosphorylation by the kinase is easily followed by the increase in fluorescence intensity over time. These probes exhibit a 3-5-fold increase in fluorescence intensity upon phosphorylation, the magnitude of which depends on the substrate. Peptides containing the reporter functionality are phosphorylated on serine by Protein Kinase C and cAMP-dependent protein kinase and are shown to be good substrates for these enzymes. The principle of this design extends to peptides phosphorylated on threonine and tyrosine.