Hybridomas secreting highly specific monoclonal antibodies against porin protein F of Pseudomonas aeruginosa were isolated. These antibodies interacted with protein F in outer membranes isolated from strains representing the 17 serotypes of P. aeruginosa and from another 15 clinical isolates from patients with cystic fibrosis. The cell surface localization of antigenic sites on protein F was shown by indirect immunofluorescent techniques with these monoclonal antibodies. No fluorescence was observed on a protein F-deficient strain H283 of P. aeruginosa. Another monoclonal antibody specific for outer membrane lipoprotein H2 of P. aeruginosa showed no fluorescence on intact, wild-type bacterial cells, but was able to interact with a rough, LPS-deficient mutant.