By an in vitro system human tears from healthy donors were exposed in a sealed vessel to a single dose of ozone. A marked destruction of tear proteins by ozone was observed as indicated by the disappearance of the protein pattern in polyacrylamide gel electrophoresis compared with the control. Lysozyme, a significant protein in tears, was demonstrated to be susceptible to degradation by O3. The destruction of tear proteins by ozone can be determined quantitatively by radial immunodiffusion as shown for human tear albumin. Variations of ozone concentrations and reaction time showed that destruction of tear proteins by ozone depended on the ozone concentration and occurred within a few minutes of exposure of tears to ozone.