Photocleavage of chicken hen egg lysozyme by three Co(III)ammine complexes, hexamminecobalt(III) chloride ([Co(NH₃)₆]⁺³), pentamminechloro cobalt(III)chloride ([Co(NH₃)₅Cl]⁺²), and tetramminecarbonato cobalt(III) nitrate ([Co(NH₃)₄CO₃]⁺), is reported here. Photocleavage resulted in two fragments of molecular masses of ∼10.5 kDa and ∼3.5 kDa which add-up to that of the parent molar mass. Detailed studies on the influence of irradiation time, excitation wavelength, the type of ligand coordinated to Co(III), concentration of the metal complex, the addition of competing metal ions, and quenchers on the protein photocleavage are reported. The Co(III) complexes also photocleaved apotransferrin, bovine serum albumin, and yeast enolase. Near-equimolar concentrations of Ni(II), Co(II) or Gd(III) inhibited the photocleavage, and therefore, binding of Co(III) metal complexes to Ni(II)/Co(II)/Gd(III) binding sites on lysozyme is necessary for the observed photocleavage. Since these ions are known to bind to Asp52 on lysozyme, we suspect that the above Co(III) complexes bind at this site, and initiate the protein cleavage. The Co(III) complexes have appropriate photochemical reactivities to cleave the peptide backbone, and they may be useful in the design of novel photochemical approaches to cleave the protein backbone.