Thromboxane B2 (TxB2) is believed to play some role in pathogenesis of interstitial pulmonary diseases. The concentration of TxB2 in BALF was assayed in 75 ill and 6 healthy persons. The ill were divided into 6 groups: with active sarcoidosis [18], with active sarcoidosis treated with corticosteroids [8], with advanced pulmonary fibrosis [15], with moderate pulmonary fibrosis [6], with avian fanciers lung after exposition of antigen [16], avian fanciers lung without exposition of antigen [13]. Radioimmunological method with the Amersham set to assay the concentration of TxB, was used. A rise of TxB2 concentration was detected in the group with advanced pulmonary fibrosis. The rise was significant compared with the group of healthy persons, those with sarcoidosis, and with moderate pulmonary fibrosis. A tendency to higher concentration of TxB2 in patients with avian fanciers lung exposed to the allergen was also observed. The results indicate that TxB2 takes part in pathogenesis of these interstitial pulmonary diseases in which increased number of neutrophils in BALF is found.