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Abstract: Background: Urinary excretion of desmosine has been reported to be increased in patients with pulmonary fibrosis; however, several investigators have pointed out that measuring urinary desmosine is not a very useful indicator of lung wall destruction. We developed a sensitive time-resolved fluoroimmunoassay (TR-FIA) to identify trace amounts of desmosine in bronchoalveolar lavage fluid (BALF), and applied this method to analyze BALF samples from healthy subjects and patients with interstitial lung diseases. Methods: In the proposed TR-FIA, a polystyrene strip was coated with desmosine-conjugated gelatin. The strip was then incubated with rabbit anti-desmosine antibody and the test solution. The desmosine bound to the solid phase and free desmosine in the sample or standard solution were allowed to compete to bind to the anti-desmosine. The solid-phase antibody was detected by Eu-complex conjugated anti-rabbit IgG. Results: The detectable limit of desmosine was 50 fmol/ml in the TR-FIA developed in this study. TR-FIA showed low cross-reactivity against amino acids. BALF desmosine levels were significantly higher in patients with idiopathic pulmonary fibrosis and sarcoidosis compared with healthy subjects. Conclusions: Desmosine levels in BALF may be useful to investigate lung disease. [Copyright &y& Elsevier]