The density of HLA-DR antigen expression on alveolar macrophages obtained by bronchoalveolar lavage from sarcoidosis patients and normal control subjects was quantified using scanning and integrating microdensitometry in conjunction with a monoclonal mouse anti-human HLA-DR antibody directly conjugated to fungal glucose oxidase. The density of HLA-DR antigen expression on alveolar macrophages was significantly increased in the pulmonary sarcoidosis patients compared with normal control subjects examined. The reproducibility, specificity and sensitivity establish the reliability and potential importance of this method of direct quantification of cell surface antigen expression. The demonstration of an increased density of Class II Major Histocompatibility Complex (MHC) antigen expression on alveolar macrophages in pulmonary sarcoidosis suggests that these cells may play a role in the induction of the immune responses at sites of disease activity in this poorly understood condition.