Crossed immunoelectrophoresis (CIE), enzyme linked immunosorbent assay (ELISA) and immunoblotting (IMB) were used to look for the presence of anti-bodies to Pseudomonas aeruginosa standard antigen in the sera of cystic fibrosis patients with intermittent (CF +/- P) and chronic (CF + P) colonization. Our results show that CIE is too complex a method to use when monitoring the infection over time, that with ELISA a correlation with the patient's clinical status may be made and that IMB can evidence an early, albeit low, reaction in the CF +/- P patient.