Two mutant proteins of human myoglobin (Mb) that exhibit altered axial ligations were prepared by site-directed mutagenesis of a cloned gene for human Mb. The normal axial ligand residue, histidine 93(F8), was replaced with cysteine or tyrosine, resulting in H93C or H93Y Mb, respectively. Cysteine or tyrosine coordination to the ferric heme iron is verified by electronic absorption, 1H-NMR, EPR spectra, and redox potentials of Fe2+/Fe3+ couple. Their mono-oxygenation activities of styrene are also discussed. © 1991 Academic Press, Inc.