Preparative electrooxidation of lysozyme at copper electrodes held at potentials around 1.2 V vs. a saturated calomel reference electrode induces the formation of a yellow chromophore with a concomitant decrease in the pI of the protein. Ion-exchange high-performance liquid chromatography revealed two new lysozyme species with pI values of 10.8 and 10.7 (lysozyme-11.0) which bear the chromophore. Sequence analysis of these two species showed that protein with lower pI was modified at both Tyr 23 and Tyr 20 and the other exclusively at Tyr 23. Ribonuclease A, subtilisin BPN’ and BSA were also found to produce the same chromophore using similar electrochemical reaction schemes. Characterization of the chromophore by a variety of techniques revealed that it is apparently 3-nitrotyrosine.