Rats were immunized systemically with various doses of the polyvalent Pseudomonas aeruginosa vaccine PEV-01. After a series of two or three doses (25 to 50 micrograms each) at 8- to 11-day intervals, animals were challenged intratracheally by the agarose bead technique with a serotype 5 P. aeruginosa strain at periods of 9 to 42 days. Immunized animals developed circulating antibodies (primarily immunoglobulin M) against vaccine components at levels significantly higher than challenged, nonimmunized controls (P less than 0.005). Eight to ten days postinfection, histological sections of lungs from immunized animals showed only minimal inflammation associated with infectious foci (agarose beads) as compared with the extensive pathological changes of airways and parenchyma seen in infected nonimmunized control animals. However, no significant reduction in bacterial numbers was observed. Such protection lasted at least 6 weeks after the final immunization. It is speculated that the vaccine may contain components of cell surface proteins and virulence exoproducts.