The effect of recombinant human urokinase (rh-UK) in a rat model of chronicPseudomonas aeruginosapulmonary infection was studied. Efficacy was assessed by lung histology and quantitative bacteriology. Male Sprague–Dawley rats received 1 × 104or 1 × 105P.aeruginosaencapsulated in agar beads via the intratracheal route on day 1. Intratracheal administration of up to 12 500 units of rh-UK on day 21 led to a dose-dependent disappearance of viable organisms from the lungs by day 24 in rats receiving 104organisms. In slightly longer term infections (30 days), rh-UK was still effective in facilitating the disappearance of the organisms from the lungs of most of the treated animals. rh-UK was effective in eliminating organisms when animals were infected with 104, but not 105bacteria. In vitro analysis revealed that rh-UK was not directly toxic for the organisms. Histologically, lungs from short-term infected control animals exhibited acute inflammation, inflammatory cell infiltrates, and fibrin deposition. Histology of lungs from UK-treated, short-term infected rats revealed decreased airway inflammation and cellular infiltration compared with infected controls. Lungs from infected animals treated with 12 500 units of rh-UK were histologically indistinguishable from the lungs of uninfected control animals, except for the foreign body reaction. These results indicate that exogenous rh-UK may be efficacious in the treatment of pulmonary inflammation accompanying exposure to Gram-negative bacteria such asP.aeruginosa.Key words: chronic pulmonary infection,Pseudomonas aeruginosainfection, fibrinolysis, urokinase.