The present invention relates to a novel trimeric OprF/I fusion protein comprising a portion of the Pseudomonas aeruginosa outer membrane protein F which is fused with its carboxy terminal end to a portion of the amino terminal end of the Pseudomonas aeruginosa out membrane protein I, wherein said portion of the Pseudomonas aeruginosa outer membrane protein F comprises the amino acids 190-342 of SEQ ID NO: 1 and wherein said portion of the Pseudomonas aeruginosa outer membrane protein I comprises the amino acids 21-83 of SEQ ID NO: 2, and further to a novel Opr F/I fusion protein which contains a disulphide bond pattern, preferably selected from the group consisting of (a) Cys18-Cys27-bond, (b) Cys18-Cys27-bond and Cys33-Cys47-bond, and (c) Cys18-Cys47 and Cys27-Cys33-bond, and to immunogenic variants thereof having at least 85% identity to the amino acid sequence of SEQ ID NO: 3. The present invention also relates to a novel method for producing said OprF/I fusion proteins and to their use for the preparation of a pharmaceutical composition and for the preparation of antibodies or antibody derivatives which specifically bind said novel OprF/I fusion proteins.