Spin labelled bovine erythrocytes have been studied by EPR spectroscopy in order to determine if exposure to infrared radiation (700-2000 nm) leads to structural changes in the phospholipid area of membranes. The results indicate that near infrared radiation strongly influences the structure of the lipid bilayer. The polarity decreases in the vicinity of polar heads. In the vicinity of the 5th carbon atom of hydrocarbon chains of the lipid bilayer the membranes are more ordered after irradiation. In the region of the 7th and 16th carbon atom disordering or generation of higher order is observed depending on the origin of the samples. For erythrocytes labelled with TEMPO-palminate and then exposed to light, the spin probe amplitude rises during the first 10 minutes of exposure to radiation and diminishes after longer time. This reverse process indicates the possibility of redox reactions on the membrane surface.