A hybrid protein [Met-Ala-(His)₆OprF_190–342-OprI_21–83] consisting of the mature outer membrane protein I (OprI) and amino acids 190 to 342 of OprF of Pseudomonas aeruginosa was expressed in Escherichia coli and purified by Ni²⁺ chelate-affinity chromatography. After safety and pyrogenicity evaluations in animals, four groups of eight adult human volunteers were vaccinated intramuscularly three times at 4-week intervals and revaccinated 6 months later with either 500, 100, 50, or 20 μg of OprF-OprI adsorbed onto A1(OH)₃. All vaccinations were well tolerated. After the first vaccination, a significant rise of antibody titers against P. aeruginosaOprF and OprI was measured in volunteers receiving the 100- or the 500-μg dose. After the second vaccination, significant antibody titers were measured for all groups. Elevated antibody titers against OprF and OprI could still be measured 6 months after the third vaccination. The capacity of the elicited antibodies to promote complement binding and opsonization could be demonstrated by a C1q-binding assay and by the in vitro opsonophagocytic uptake ofP. aeruginosa bacteria. These data support the continued development of an OprF-OprI vaccine for use in humans.