Recombinant outer membrane protein F of Pseudomonas aeruginosa was purified by extraction from polyacrylamide gels of cell envelope proteins of an Escherichia coli strain expressing the cloned gene for protein F. Rats were immunized intramuscularly with 25-mu-g of recombinant protein F adsorbed to aluminum hydroxide adjuvant on days 1, 14, and 28 and then challenged on day 42 via intratracheal inoculation of agar beads containing cells of a clinical isolate of P. aeruginosa. On day 49 the lungs were examined macroscopically for the presence and severity of lesions and submitted for quantitation of the bacteria present. The recombinant protein F vaccine afforded significant protection against subsequent challenge with P. aeruginosa in the immunized rats, as compared with control rats immunized with bovine serum albumin. Antisera from the recombinant protein F-immunized rats mediated opsonophagocytic uptake by human polymorphonuclear leukocytes of wild-type cells of P. aeruginosa but exhibited no opsonic activity against a protein F-deficient mutant of P. aeruginosa. The antisera to recombinant protein F did not promote complement-mediated bacteriolysis of P. aeruginosa. These data demonstrate that recombinant P. aeruginosa protein F has efficacy as a protective vaccine in a rat model of chronic pulmonary infection.