Enhanced expression of CD2 antigen on lung T cells. CD2 molecules not only function in the adhesion of T cells to other cell types but also in the activation of T cells via an alternative pathway. To investigate the possible roles of CD2 molecules in the activation and accumulation of lung T cells, using monoclonal antibodies and a flow cytometer we evaluated CD2 antigen expression on lung and blood lymphocytes in 10 normal subjects, 30 patients with pulmonary sarcoidosis, 7 patients with interstitial pneumonia associated with collagen vascular disease, 5 patients with farmer's lung disease, and 8 patients with Crohn's disease. The mean fluorescence intensity (MFI) of CD2 on lung T cells obtained by bronchoalveolar lavage (BAL) was significantly higher than that on blood T cells in all study groups except the control group. In patients with pulmonary sarcoidosis this enhancement of the expression of CD2 antigen was observed only on CD4+ T cells. Since ligands for CD2 are present on immunocompetent cells of other types in the local pulmonary milieu, these results suggest that increased expression of CD2 molecules could facilitate the communication and interaction of lung T cells with their environment in the lung and thereby possibly contribute to the local accumulation of T cells. 