A simplified solid-state enzymatic iodination procedure for routine labeling of unstable pure protein or complex amino acid-containing molecules is presented. The procedure was designed using agarose-bound lactoperoxidase to iodinate human IgG with iodine-125. This method consistently resulted in a labeling efficiency greater than 90% with high stability and undetectable gross structural alterations of the substrate as evaluated by immunodiffusion and electrophoresis. The technique presented is simple, efficient, and may be employed to yield a sterile, pyrogen-free labeled species.