Phosphoramidon (N-(alpha-L-rhamnopyranosyloxyhydroxyphosphinyl)-L-leucyl-L-trypto phan) is a powerful inhibitor of Pseudomonas aeruginosa elastase. The addition of this compound to an elastase solution injected intrastromally in rabbit eyes protected the corneas from the damage of the enzyme for a period of 12 hours. Phosphoramidon is comparable in this respect to the inhibitor 2-mercaptoacetyl-L-phenylalanyl-L-leucine, but is considerably more effective than its analog phosphoryl-L-leucyl-L-phenylalanine (P-Leu-Phe). It is suggested that the rhamnopyranosyl moiety, present in phosphoramidon but not in P-Leu-Phe, is responsible for the difference in the intracorneal activity of the two phosphoramidates. Phosphoramidon as well as the mercaptoacetyl derivative might prove beneficial in the treatment of Pseudomonas aeruginosa corneal infections.