Protoporphyrin IX, a naturally occurring precursor of hemoglobin found in serum and erythrocytes, is a photoactive compound that we have detected in aqueous aspirates of three patients with hyphemas (0.13, 0.33, and 0.67 microgram/dL). To determine whether corneal endothelial photosensitization could occur, rabbit corneas were exposed to physiologic concentrations of protoporphyrin and light (200 microEinsteins [microE]/m2 per second). Experimental corneas demonstrated endothelial swelling with loss of intercellular contact by scanning electron microscopy. Flux chamber experiments demonstrated an increase in the net endothelial inflow, indicating a loss of the endothelial barrier function. Control corneas had normal ultrastructural architecture and flux study results. These studies suggest that patients with long-standing hyphemas who may be exposed to long periods of light are possibly at risk for developing endothelial dysfunction and corneal blood staining. Patching of these patients' affected eyes may be prudent.