The relative reactivity of the tyrosine side chains in the proteins of skeletal muscle myofibrils was determined using iodination techniques. The destruction of ATPase activity of myofibrils and myosin by lactoperoxidase and chloramine-T iodination could be prevented by the attachment of cysteamine to the sulphydryl groups prior to the iodination reaction and subsequent regeneration with thioglycolate or dithiothreitol. Iodination using 1,3,4,6-tetrachloro-3 alpha, 6 alpha-diphenylglycoluril did not require cysteamine treatment for retention of full enzymatic activity. The specific activity of the different proteins varied markedly with desmin, troponin-T, and tropomyosin having the highest labelling with all three iodination procedures. In contrast the myosin light chains had low specific activity when labelled in myofibrils or intact myosin. The isolated light chains, however, were much more highly iodinated. It appears that iodination may be a useful technique for examining protein-protein interactions in the myofibril.