This study examined CD11/CD18-mediated adhesion in neutrophil emigration during acute and recurrent Pseudomonas aeruginosa-induced pneumonia. Neutrophil emigration during acute pneumonia was studied in anti-CD18 antibody or murine-IgG-pretreated rabbits 4 hours after intrabronchial instillation of P. aeruginosa. To examine emigration in recurrent pneumonias, rabbits given P. aeruginosa on day 0 received anti-CD18 antibody or IgG on day 7. A second instillate was placed either at the initial site or in a separate lobe, and emigration into alveolar spaces was quantitated morphometrically after 4 hours. The results show that CD11/CD18 was required for neutrophil emigration in acute pneumonias and in recurrent pneumonias that occurred at a site distant from the initial infection. However, when the recurrent pneumonia occurred in the previously inflamed site, CD11/CD18 was not required. When the same number of organisms were instilled on days 0 and 7, emigration was reduced to 15 to 20 percent of the number that migrated initially and only CD18-independent adhesion pathways were used. Increasing the concentration of organisms threefold increased emigration through both CD18-dependent and CD18-independent pathways. These data indicate that P. aeruginosa induces CD11/CD18-dependent emigration during acute pneumonia and recurrent pneumonia at previously uninflamed sites. However, adhesion pathways are altered in regions of chronic inflammation, and a greater proportion of neutrophil emigration occurs through CD11/CD18-independent pathways.