The immunological basis of the inflammatory response in the lungs of patients with pneumonitis after bone marrow transplantation has been investigated by means of bronchoalveolar lavage. Ten episodes of pneumonitis associated with cytomegalovirus and nine episodes due to various other infectious and non-infectious causes were investigated in 16 patients (three patients had two episodes of pneumonitis). Total lavage cell counts and differential cell counts were determined and compared with results from normal control subjects. In most patients with pneumonitis the total cell yield was greater than normal (mean 6.8 (SD 6.0) x 10(5) cells/ml; normal 1-2 x 10(5) cells/ml). The percentage distribution of these cells was 71.9 (17) macrophage like cells, 24.1 (15.8) lymphocytes, 5.0 (5.0) polymorphonuclear cells, and 0.7 (1.0) eosinophils. None of the patients had peripheral lymphocytosis despite the increased number of lymphocytes in the lavage fluid. Further analysis of the lymphocyte population using monoclonal antibodies with immunocytochemical techniques showed that B cells were generally present in normal proportions, whereas the proportion of cells expressing T lymphocyte markers (CD2+, CD5+, CD8) were reduced in nine out of 19 cases. In 10 of the 19 episodes there were substantial numbers of cells expressing none of the B or T cell antigens studied ("null" cells). These abnormalities bore no relation to survival. The total cell yield, the proportion and number of lymphocytes, and the proportion and number of T cells in the bronchoalveolar lavage fluid were all lower in the group with cytomegalovirus infections than in those with pneumonitis from other causes. These results suggest that the pneumonitis in recipients of bone marrow transplants is associated with a local immune response despite the fact that the individuals are otherwise immunosuppressed.