Burkholderia cepacia has become a serious source of infection in patients with cystic fibrosis. Antibiotic therapy is difficult as the bacteria are intrinsically resistant to most antibiotics. The present study compared the antibody response by immunoblot of 50 negative control sera, 22 patients with cystic fibrosis and no evidence of B. cepacia, 9 clinically well patients with cystic fibrosis colonised by B. cepacia and 5 patients with cystic fibrosis and deteriorating or fatal B. cepacia infection. Nineteen antigenic bands varying in apparent molecular weights from 19 to 170 kDa were identified. Two bands at 19 and 21 kDa were only present when the organism was grown in an iron-deficient medium. The band at 30 kDa was identified as a porin and the possession of IgG antibody carried a statistically significant (P = 0.00003) better prognosis. This antigen was thus a potential target for immunotherapy.