 Anti-pseudomonas IgY is prepared from eggs of hens that have been vaccinated with Pseudomonas aeruginosa. The hens farm is under veterinary control according to Swedish rules. The drug is prepared with a water dilution method according to GMP standards approved by Swedish MPA. Patients with CF who are intermittently but not chronically infected with P. aeruginosa get a short course of antibiotics to eradicate the bacteria. Thereafter they start to gargle with a solution of anti-pseudomonas IgY every night to prevent a new infection. Preliminary results have shown that it takes a significantly longer time to get a new infection and that the patients get fewer infections than control patients. In addition, results tentatively indicate that: patients have not got any new opportunistic bacteria or fungi B. Cepacia, S. Maltophilia, A. Xylosoxidans, atypical Mycobacteria, Aspergillus Fumigatus; that use of antibiotics is greatly diminished; that the lung functions and nutritional conditions are maintained. 