The distribution and degree of expression of Class I and Class II major histocompatibility (MHC) antigens on human lower respiratory tract epithelium were evaluated in five freshly obtained pneumonectomy and lobectomy specimens using an immunoperoxidase technique. Multiple sites were examined from each specimen, and two independent observers graded each sample as positive, equivocal, or negative compared with control slides. Interobserver agreement was high. From a total of 120 grade determinations, 114 showed complete concordance and only one showed a positive/negative discordance. Both Class I (HLA-A,B,C) and Class II (HLA-DR) antigens were uniformly and strongly expressed throughout the major, lobar, and segmental bronchi of each sample, the bronchiolar epithelium, and the alveolar epithelium. Paired samples of adjacent lower respiratory tract epithelium harvested with the fiberoptic bronchoscope and during pathologic examination, respectively, revealed an identical staining pattern for these antigens. Staining for HLA-DQ expression (a subset of MHC Class II antigens) was generally weaker and appeared more variable, with four negative, six equivocal, and 30 positive samples. Our observations demonstrate the widespread expression of Class I antigens on airway epithelium and reveal for the first time the ubiquitous nature of Class II MHC antigen (HLA-DR) expression throughout the lower respiratory tract. Furthermore, they attest to the adequacy of bronchoscopically obtained samples for immunologic staining. These results provide a basis for both a putative mechanism of bronchocentric rejection phenomena after human heart-lung transplantation and for the means to monitor it prospectively.