Induction of lipid peroxidation in tissues or cells produces fluorescence, crosslinks and borohydride-reducible functions in the proteins. Fatty acid hydroperoxides, initial products of lipid peroxidation, produce fluorescence and cross-links by reacting with proteins, and the fluorescence characteristics of the products are similar to those of the products formed in cells and tissues. Malonaldehyde that may be generated to a little extent during lipid peroxidation produces fluorescence, cross-links and borohydride-reducible functions in proteins, but the fluorescence characteristics of the products are not always similar to those in cells and tissues. Other carbonyls that are generated during lipid peroxidation may be responsible for the formation of the damaged proteins in cells and tissues. Modified proteins or the cells that contain the modified proteins may be removed by several mechanisms. These processes of removal may link to certain abnormal disorders so called “free radical disorders”.