iso-1-Cytochrome c from Saccharomyces cerevisiae is able to oxidize polycyclic aromatic hydrocarbons (PAH) in the presence of hydrogen peroxide. Anthracene and pyrene are oxidized by yeast cytochrome c to form anthraquinone and 1,8-pyrenedione, respectively. Iso-1-cytochrome c from S. cerevisiae was modified by site-directed mutagenesis of Phe82 and Cys102. The Phe82 substitution significantly altered the kinetic behavior of the protein; Cys102 modification affected neither the kinetic nor the stability constant. The Gly82;Thr102 valiant was 10 times more active and showed a catalytic efficiency 10-fold greater than the wild-type iso-1-cytochrome c. However, Phe82 variants showed lower stability against inactivation by hydrogen peroxide than the wild-type protein. These site-directed mutations did not significantly alter the stability and activity of the hemoprotein in increasing concentrations of tetrahydrofuran.