Immune complexes (IC) were investigated in the bronchoalveolar lavage fluid (BAL) of 5 patients with hypersensitivity pneumonitis (HP), 11 with idiopathic pulmonary fibrosis (IPF) and 16 with sarcoidosis (S) by three different methods: C1Q-BA, KgB, AKgB-MA. Using AKgB-MA, it is possible to identify the class of antibodies forming the IC. IC were present in all cases of HP, in 8/11 (73%) of IPF and in 10/16 (62%) of S. However, the three tests showed discordant results for the three different diseases: C1Q-BA and KgB-SP were both positive in 40%, AKgB-MA in 80% of HP cases; C1Q-BA in 73%, KgB-SP in 9% and AKgB-MA in 46% of IPF cases; C1Q-BA in 31%, KgB-SP in 12% and AKgB-MA in 31% of S cases. In all of the diseases, the IC were mostly composed of IgG; moreover, in HP IgA was also frequently present. The determination of IC in different fractions obtained from BAL ultracentrifugation, confirmed the simultaneous presence of IC of different molecular weights and antibody composition. Lung transbronchial biopsy with immunofluorescence showed immunoglobulin, prevalently IgG, and C, in all HP cases, the majority of IPF cases and 50% of S cases. This confirms the importance of IC in the pathogenesis and/or evolution of some pulmonary diseases.