In 12 patients with sarcoidosis and 11 patients with allergic alveolitis concentration of procollagen III peptide in bronchoalveolar lavage fluid (in bronchial and alveolar fraction) was estimated using RIA method. In studied populations procollagen III levels were higher in comparison to control. In patients with allergic alveolitis and with DLCO < 60% pred. procollagen III peptide concentrations in BAL fluid were significantly higher than in patients with sarcoidosis. In patients with allergic alveolitis a positive correlation between BAL lymphocytes number and procollagen III peptide concentration was observed.