Corneal rings have been described with corneal ulcerations caused by Gram-negative bacteria. Corneal rings were produced by intracorneal injections of viable Pseudomonas aeruginosa as well as heat-inactivated suspensions of Gram-negative bacteria (P aeruginosa and Escherichia coli) but not Gram-positive bacteria (Staphylococcus aureus) or Freund's adjuvant. It is suggested that endotoxin is the factor responsible for their production since purified endotoxin produced corneal rings after intracorneal injection. Histopathological examination of the areas corresponding to the corneal rings disclosed that the rings represented accumulations of polymorphonuclear leukocytes. Direct immunofluorescent studies of the corneal rings revealed staining for properdin and C3 complement but not for immunoglobulins. The present report suggests that endotoxin has the ability to stimulate the alternate pathway of complement through properdin activation with the production of chemotactic fragments that attract polymorphonuclear leukocytes into the cornea.