Sulfonated phenyl isothiocyanate cytochrome c is suggested as a new scavenger for superoxide anion. The efficiency of the modified cytochrome c in measurements is compared with that of phenyl isothiocyanate cytochrome c and acetylated cytochrome c. Sulfonated phenyl isothiocyanate cytochrome c is water and salt soluble. Autooxidability of the pigment is not observed. The primary advantage of sulfonate phenyl isothiocyanate cytochrome c is that it appears to be specifically reduced by O2 radicals without interferences by other reactions in complex biological systems.