SUMMARY— Reflectance spectra of cooked meat and the precipitates obtained on heating aqueous muscle extracts, mixtures of bovine serum albumin (BSAl and horse heart metmyoglobin IMbl at 60°C. and Mb alone at > 80°C were all similar. BSA-Mb mixtures in 5.5 M urea atpH 6.0 yielded soluble complexes that were spectrally the same as those present in cooked meat. Sephadex chromatography showed these complexes to be denatured hemoproteins where the protein could be denatured BSA or dimers or higher aggregates of denatured apomyoglobin (ApoMb). The affinity of the hematin for denatured BSA was much greater than for the ApoMb aggregates. No evidence was obtained for complexes of a type where hematin was coupled with both denatured ApoMb and denatured BSA. It is suggested that the complexes present in cooked meat are also denatured hemoproteins, where the protein may be any of several of the denatured proteins present in cooked meat. An electron-spin resonance (ESR) spectra of cooked meat indicated that at - 196°C the ferric ion in the hematin was essentially low-spin. The mechanism of formation and the nature of the possible bonding between hematin and denatured protein are discussed.