The alveolar macrophage (AM) plays an essential role in the pathogenesis of interstitial lung diseases. The expression of specific membrane antigens is related to the functional or madurative status of the cells of mononuclear phagocyte system. The aim of this study was to analyze the expression of several markers (HLA-DR, CD11b, CD16, CD14) in AM obtained by bronchoalveolar lavage from control patients (n = 6), patients with sarcoidosis (n = 6), diffuse neoplastic infiltration of the lung (n = 7), pulmonary fibrosis (n = 4), and hypersensitivity pneumonitis (n = 3) by two evaluation techniques (flow cytometry and alkaline immunophosphatase). In the light of the results we can conclude that in the immunophenotypical study of the alveolar macrophage, flow cytometry (with semiquantitative evaluation to avoid the problem of autofluorescence) is a useful tool in the evaluation of those antigens that are only weakly or moderately expressed on AM (CD11b or CD14), whereas the alkaline immunophosphatase technique is of great interest in the evaluation of those that are strongly expressed (i.e., HLA DR). Additionally, the variable expression of the different antigens in the different alveolar-interstitial pathological states is patent in some diseases.