Cells of Pseudomonas aeruginosa were titered (1.0× 104–1.0× 108 cells) to determine the minimum number required to initiate corneal infection in Swiss-Webster and BALB/c mice. No infection was produced in either strain of mouse by direct application of the organisms to unwounded corneas. However, wounding the cornea in either strain followed by application of the bacteria (1 × 107–1 × 108 cells) produced corneal infection within 18–24 h. In Swiss-Webster mice, corneal morphology appeared normal within 4–6 weeks. However, in BALB/c mice, changes occurred by 12–15 days post-infection resulting in micro-ophthalmia. The results suggest development of an experimental model to comparatively study the natural immune mechanism of the eye to bacterial challenge.