Disclosed are assays useful for detecting kinase activity. The assays utilize thiol-reactive fluorescent reagents and include reacting a reaction mixture that contains an ATP analog such as ATP.beta.S to form an ADP analog such as ADP.beta.S if kinase activity is present in the reaction mixture. The ADP analog reacts with the thiol-reactive fluorescent reagent and the reagent exhibits a change in fluorescence, thereby indicating that kinase activity is present in the reaction mixture.