Although the histologic manifestation of cytomegalovirus (CMV) is usually characteristic intracellular inclusions and cytomegaly, some investigators, using immunohistochemical or in situ hybridization techniques, have demonstrated the presence of histologically occult infections in certain tissues. A series of lung biopsy specimens from pulmonary transplant recipients were studied using a monoclonal antibody (CCH2) to CMV early viral antigen and the results were compared with routine histologic findings. Occult infection could not be demonstrated in any of these cases. These results may reflect the relative sensitivity of the monoclonal antibody used in this study, although other possible factors are discussed. The results suggest that, in lung allograft biopsy specimens, immunohistochemical analysis using monoclonal antibody CCH2 is not likely to increase significantly the yield of positive cases compared with examination of multiple levels of hematoxylin-and-eosin-stained sections. Additional studies are needed to compare the sensitivity of monoclonal antibodies to CMV antigens using a variety of sampling techniques and clinical settings.