The reductive metabolism of BrCCl3 by ferrous myoglobin leads to the alteration of the prosthetic heme to form products that can be dissociated from the protein and to those that are irreversibly bound to the protein. The major dissociable or soluble heme metabolites have recently been characterized. In this study, the irreversibly bound heme product was characterized by Edman degradation, amino acid analysis, and electronic absorption and mass spectrometry of peptides derived from the altered protein. It was found that the prosthetic heme was modified by a CCl2 moiety derived from BrCCl3 and was covalently bound to histidine residue 93, the normal proximal ligand to the heme-iron. The data are consistent with a mechanism by which the trichloromethyl radical reacts with the heme to form an intermediate that either can alkylate the proximal histidine residue or form soluble metabolites. The covalent bonding of the heme prosthetic moiety to the apoprotein likely leads to a change in the tertiary structure of the protein that may be responsible for its altered catalytic activity as well as its enhanced susceptibility to proteolysis. Similar processes may account, at least in part, for the covalent alteration of the heme prosthetic group of other hemoproteins caused by xenobiotics and endogenous substrates.