Light damages the cultured bovine retinal epithelium at about the same intensities as those effective in the primate retina, in vivo. Also as in vivo, blue light (457.9 nm) is substantially more damaging than longer wavelengths of the visible spectrum. Experiments were performed to test whether this damage by light is mediated by a photodynamic reaction. Primary confluent monolayer cultures of the bovine retinal epithelium were exposed to 435 nm light for 120 minutes, during which time the oxygen content in the gas phase of the exposure chamber was maintained at a predetermined value. Prior to and after light exposure the cultures were aerated by 20% oxygen. The normal growth medium (Eagle's MEM + 10% fetal calf serum) was used during exposure. Light microscopic and EM examination 24 hours after exposure showed that changes indicating cell death were enhanced by a factor of 10 when exposures were conducted in an atmosphere of 95% oxygen instead of 20% oxygen. No damage at intensities up to 50 mw/sq.cm. was observed with a 0% oxygen atmosphere. These effects were the same when a saline, bicarbonate buffered medium was used during exposure. It is concluded that short wavelength light damage to the retinal epithelium is caused by a photodynamic reaction, i.e. a photosensitized aerobic oxidation. This is the first time that the mechanism of blue light damage on a retinal tissue has been established.(ABSTRACT TRUNCATED AT 250 WORDS)