The prophylactic effect of pili in prevention or amelioration of Pseudomonas aeruginosa ocular disease was examined in mice, using both systemic and topical protection approaches. At 30 days postinfection, a significant number of animals actively or passively immunized with pili homologous to pseudomonal strain American Type Culture Collection 19660 were protected from ocular disease when compared with similarly infected phosphate-buffered saline controls. Although exogenously mixing strain 19660 with either homologous or heterologous (PAK/PR1) pili before topical application of the inoculum significantly inhibited bacterial adhesion in vitro, in similarly designed in vivo studies, animals were not protected from corneal disease. Neither was significant ocular protection conferred using pili (PAK/PR1) heterologous to the infecting strain (19660) for active or passive immunization of mice, nor in studies involving exogenous mixing of PAK/PR1 pili or its pili-specific monoclonal antibody with strain 19660 prior to topical application of the latter. These results provide evidence that significant ocular protection is achieved by either active or passive, but not topical, immunization with pili homologous to the infecting bacterial strain and that immunization with pili heterologous to the infecting bacterial strain fails to provide protection against ocular disease, despite the fact that heterologous pili are highly effective at decreasing bacterial binding to cornea in vitro.