abstract: Chlorophyllin a (CHLA), a food-grade derivative of chlorophyll a, is a potent inhibitor of carcinogenesis. Recently a growing interest in CHLA application as a natural colorant and diet stabilizing supplement is observed. The study presented herein is concerned with the interaction between CHLA and ferricytochrome c (ferriCY) in aqueous solution at physiological conditions, studied by means of spectroscopic methods - absorption and fluorescence in the visible range and ultraviolet circular dichroism (UV-CD). In a 0.1M solution of phosphate buffer (pH 7) CHLA undergoes dark transformation from 645 nm (red) to 735 nm (far red) absorbing form with half time of 16×103 s. This first order reaction is activated by ruby laser irradiation energy. FerriCY with CHLA in solution undergoes reduction and forms a complex with an additional band of 694 nm in the absorption spectrum. The data indicated 1:1 stechiometry of ferriCY-CHLA complex formed at single bond site with association constant KA = 1.1×106 M-1 and free energy DG= -33.8 kJ mol-1. In such a complex UV-CD spectra reveal a decrease in the a-helix structure from 22.4% (native ferriCY) to 16.05%. These studies indicate that CHLA, when binding with a protein, is an effective reductant (antioxidant).