A preliminary diagnosis of tuberculosis can be established by the detection of acid-fast bacilli (AFB) and confirmed by culture of the microorganism. To evaluate an alternative method of diagnosis, the distribution of mycobacterial antigens in lung tissue specimens was characterized by an indirect peroxidase-antiperoxidase method and was compared to the detection of AFB by Ziehl-Neelsen stain. Histologic specimens were obtained from 59 hospital patients. Of nine patients with mycobacterial disease, seven had antigen detected in tissue. In two patients with tuberculous pneumonia, the distribution of mycobacterial antigens was approximately the same as that of AFB. In contrast, in four patients with caseating pulmonary granulomas, clumps of mycobacterial antigens were demonstrated in necrotic areas of the granulomas where there were few or no AFB. In one patient with Mycobacterium intracellulare infection, cross-reactive antigens stained weakly. Antigen was not found in tissue from two patients; one had miliary lung granulomas, and the second had mediastinal lymph node granulomas. Mycobacterial antigens were not detected in specimens from 50 control patients with nonmycobacterial diseases. On the basis of this study of 59 cases, immunohistochemical detection of microbial antigens appears to be useful for establishing the mycobacterial etiology of caseating pulmonary granulomas.