Bronchoalveolar lavage (BAL) was carried out in six patients with radiation pneumonitis, the early radiation-induced lung damage that usually leads to radiation fibrosis. Protein analysis by immuno-electrophoresis and polyacrylamide gel electrophoresis of BAL fluid revealed leakage of circulatory proteins, including high molecular weight components. Cell count of BAL fluid showed an increased number of lymphocytes; these proved to be activated on cell cycle analysis in one patient. Collagenolytic activity, assessed by degradation of radiolabelled type I human collagen, was present in BAL fluid of all six patients. The following mechanisms are therefore considered to participate in the pathogenesis of radiation-induced lung damage: 1) permeability oedema, which led to acute respiratory distress syndrome in one case of hyperacute radiation pneumonitis, 2) lymphocyte alveolitis possibly perpetuated by activated lymphocytes, and 3) release of collagenolytic enzymes in alveolar structures contributing to fibrotic processes.