S-aminoethylcysteine ketimine and other sulfur-containing similar ketimines reduce molecular oxygen and phospho-18-tungstate (Folin Marenzi reagent), although the sulfur atom is formally present in the non reducing thioether form. We have now found that 2,6-diclorophenolindophenol, some ferrihemoproteins and other reagents are also reduced by this group of ketimines. Ferricytochrome c is reduced faster than methemoglobin, metmyoglobin and free hematin, whereas horse radish peroxidase compound I is reduced at once. These results indicate a wider reducing activity of this type of ketimine. The oxidation of ketimines by ferric cytochrome c appears a relevant finding pointing to a new possible way of enzymatic modification of sulfur-ketimines in tissues.