We have used the absorption spectra of whole blood, erythrocytes, and plasma to study photochemical reactions initiated by exposure of blood in vivo to UV radiation (UV irradiation) in the UV-visible and IR regions of the spectrum. We have established that when blood is exposed to therapeutic doses of UV radiation (0.5 J/cm²), the absorption of blood proteins decreases as monitored using the UV absorption and luminescence bands of the proteins; photochemical reactions are initiated in the protein and heme components of the hemoglobin. For the studied doses, the reversible reaction of photodissociation of hemoglobin complexes with oxygen is one of the most likely primary reactions initiated by UV irradiation of blood. We conclude that changes in the position and relative intensities of the IR absorption bands of the peptide groups (stretching and bending vibrations of NH, CN, and CO bonds) may be due to conformational transitions in the blood protein macromolecules, induced with a change in the intermolecular hydrogen bonds on absorption of the UV radiation by the blood. The changes in the absorption spectra of blood initiated by UV irradiation are compared with the results of laboratory blood analyses.