The expression of MHC class II antigens and ICAM-1 and the composition of lymphocyte infiltrates have been studied in frozen sections of transbronchial biopsies from lung transplant recipients. First, biopsies obtained from patients who showed acute rejection, OB, and normal features were compared. Second, we compared first-year biopsies from patients developing OB and patients with a good clinical outcome. HLA-DR was widely expressed on epithelia and vascular endothelium. Increased vascular HLA-DP expression was found in OB biopsies. In OB patients there was a significantly increased frequency of bronchial HLA-DP and vascular HLA-DQ expression. Expression of ICAM-1 by bronchial and bronchiolar basal cells, a phenomenon not reported previously in humans, was seen in a small number of biopsies. CD8 predominant lymphocytic infiltrates were present in all groups and were increased in OB biopsies and OB patients. Increased numbers of CD4-positive cells were found in rejection and OB when compared with normal biopsies. These findings support an immunological basis for the development of OB.