Using a mouse monoclonal antibody raised against recombinant human tumour necrosis factor alpha (TNF alpha), we have studied immunostained paraffin-embedded autopsy lung tissues from 13 patients dying with adult respiratory distress syndrome and 10 control patients dying of unrelated conditions. Material from 12 of the 13 cases showed positive staining, strong in six, less strong in the remaining six. TNF alpha was located principally within epithelial cells resembling type II pneumocytes in both early and late stages of adult respiratory distress syndrome. In the early stages the TNF alpha positive cells were scattered and appeared disorganized, but in later cases they formed an epithelium. The cells were confirmed as epithelial by their staining pattern with monoclonal antibodies reactive with epithelial, cytokeratin and macrophage antigens, and by double staining for epithelial membrane antigen and TNF alpha. In the control group, staining for TNF alpha was confined to very occasional plump type II pneumocytes, and a weak reaction in bronchial lining cells. Macrophages were only weakly stained in either group. It is suggested that TNF alpha is synthesized (or possibly absorbed) by type II pneumocytes and may be involved in the production of, or recovery from, adult respiratory distress syndrome.