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Two pathogenic mechanisms are discussed as possibly being responsible for destruction of the corneal tissue in the course and corneal infections caused by Pseudomonas aeruginosa. One involves bacterial exoenzymes and the other polymorphonuclear (PNM)-derived lysosomal enzymes. The objective of the present study was (1) to show the relative importance of the two mechanisms and (2) to evaluate the effect of active immunization against Pseudomonas aeruginosa exoenzymes on ocular damage. Rabbits were immunized against Pseudomonas aeruginosa alkaline protease (AP) or elastase (Ela) and challenged with the respective enzymes. Corneal damage was studied by light photography. In another group rabbits were immunized against AP, Ela and exotoxine A and challenged with Pseudomonas aeruginosa strain PA 01 und PA 103. Corneal damage was studied by light photography, light microscopy and electron microscopy. Immunized animals were totally protected against purified Pseudomonas aeruginosa exoenzymes. Less corneal damage was found in immunized rabbits than in non-immunized ones. 12 and 24 h after infection with whole bacteria. However, after 48 h corneal damage was comparable in both groups. These results show that corneal damage by lysosomal enzymes from stimulated PMN ist more important in Pseudomonas aeruginosa than direct damage by bacterial exoenzymes.