A kit and method of non-destructively radiolabelling a biomolecule substrate by halogenation, including the steps of: a) reacting a secondary amine with a chlorinating agent to produce an N-chloro secondary amine, (b) reacting the N-chloro secondary amine with a salt of a radiolabelled halogen to produce a radiolabelled halogenating agent, (c) reacting a solution of the biomolecule substrate with the radiolabelled halogenating agent, and (d) recovering a radiohalogenated biomolecule from the reaction of step (c). The kit and method allow for more complete iodination with a lessening of side reactions such as decomposition of the biomolecule substrate to be labelled
