Photooxidation is a method of tissue fixation in the construction of bioprostheses. This process leads to the tissue protein crosslinking under light and dye. In the present study, the influence of methylene blue and methylene green on the effectiveness of pulmonary valve and aorta collagen modification was evaluated on the basis of photooxidized tissues sensitivity to pepsin digestion. The components of tissue hydrolysates were characterized electrophoretically. The tissues photooxidized in the presence of employing dyes showed increased resistance to enzymatic degradation in comparison to the unfixed ones. However, methylene green was more effective in valve and aorta collagen crosslinking. Hydrolysates of the tissues fixed in the presence of mentioned dye displayed decrease in collagen chains and their aggregates? content.