Abstract: A method for assaying for enzymes that modify peptide chains, such as protein kinases and enzymes which modify the C-terminus of the Ha-RAS protein, is defined. This is done by incubating an extract in which the enzyme being assayed for may be present contained in a reaction mixture. The reaction mixture is made up of a buffer solution, a ubiquitin peptide extension, wherein the peptide contains a sequence known to be modified by an agent in the presence of the enzyme being assayed for, and the agent known to modify the peptide extension when the enzyme is present. The incubation is stopped and the ubiquitin peptide extension is separated from the solution and analyzed for the presence of the agent modified peptide. The extent of peptide modification can be both qualitative and quantative of the enzyme being assayed for. Protein kinases can be assayed for using a ubiquitin pepide extension containing the sequence (SEQ ID NO:1), Ser-Glu-Glu-Glu-Glu-Glu in the presence of a phosphorylating agent.