For an animal model of infection to be useful in immunological studies it is necessary to establish that the surface antigens expressed by bacteria growing in vivo in the experimental infection mimic those expressed by bacteria in the human infection. In this study, chronic infection was induced by inoculating the lungs of rats with agar beads containing mucoid Pseudomonas aeruginosa. P. aeruginosa was obtained from the lungs 14 days after infection and studied without subculture. Several high-mol.-wt proteins were expressed in the outer membranes (OM) of the bacteria from the rat lungs which could be induced by cultivating the same isolate in iron-depleted conditions in vitro. The pattern of iron-regulated membrane proteins (IRMP) was similar to that obtained in an earlier study with another mucoid isolate of P. aeruginosa examined directly, without subculture, from the sputum of a cystic fibrosis patient. Immunoblotting with LPS-absorbed serum from infected rats and also with serum from CF patients showed that IgG in these fluids reacted with the IRMPs and other major OM proteins (OMPs) of P. aeruginosa. Antisera from rats immunised with whole cells of P. aeruginosa grown in iron-depleted media reacted with all the major OMPs of P. aeruginosa, including the IRMPs, confirming their immunogenicity.