Pseudomonas aeruginosa seldom invades the body except in persons or in organs lacking natural defenses, and usually the infection is chronic rather than acute, evoking little systemic response. When introduced into the cornea, however, as in penetration by a foreign body or in contaminated medicines, it acts with extreme virulence, in many cases causing blindness and even necessitating enucleation. Although many attempts at control of Ps. aeruginosa, even with powerful antibiotics, have been unsuccessful, polymyxin B appeared to have good effect and was tested in experimental infection of the cornea in rabbits. It was demonstrated by preliminary studies in vitro that polymyxin B was effective against nine strains of Ps. aeruginosa which on inoculation caused rapidly progressive ulcers in the corneas of rabbits.A strain of proved virulence was introduced into both eyes of each of 18 rabbits. The left eyes only were treated with subconjunctival injections at 48-hour intervals of a solution of polymyxin B, to which epinephrine was added as a vasoconstrictor to prevent rapid dispersion. The right eyes remained untreated as controls. In five of the six rabbits treated immediately after inoculation, the treated eyes remained clear, while moderate infiltration developed in the sixth. In the six rabbits not treated for 24 hours after inoculation, ulcers developed but remained localized during therapy. In those not treated for 48 hours after inoculation, ulcers developed before treatment began but did not spread as rapidly as in the controls. Hyaluronidase was added to the preparation for half the rabbits in each group but had no perceptible beneficial effect.