The immunological response to Ps. aeruginosa antigens may be a sensitive and early indicator of the colonisation of the lungs with these organisms in patients with cystic fibrosis. This study used an immunoenzymatic system and immunoblotting to evaluate the antibody response of 20 patients without apparent Ps. aeruginosa lung infection. These was an agreement in the results obtained with the two methods in 87.5% of cases.