Wild-type sperm whale myoglobin (WT Mb) and L29F mutant were used to examine the effect of metMb formation rate on Mb-mediated lipid oxidation in washed cod muscle. MetMb formation was 15-fold slower in L29F compared to WT Mb at 2 degrees C (pH 5.7). The electrostatic interaction of bound O(2) and the partial positive edge of the phenyl ring of phenylalanine(29) inhibits deoxyMb-mediated metMb formation and may displace protons that promote oxyMb oxidation. Ferrous L29F was a poor promoter of lipid oxidation in washed cod during extended storage, whereas ferrous WT Mb oxidized the substrate readily. This diminishes a role for free radicals produced by the reaction of oxyMb with peroxides. MetL29F was an effective promoter of lipid oxidation. Mb mutants with low hemin affinity (H97A) were better promoters of microsomal lipid oxidation than mutants with higher hemin affinity (WT Mb and V68T). The much higher heme affinity of oxyMb compared to metMb partly explains the poor ability of ferrous L29F to oxidize lipids in washed cod at post-mortem pH during 2 degrees C storage. The relative roles of cross-linked Mb and hypervalent Mb species to promote lipid oxidation in washed cod at post-mortem pH are discussed.