A murine monoclonal antibody to cytomegalovirus (CMV) was used to identify virus-infected cells in coded frozen tissue sections from 52 consecutive open-lung biopsies obtained from marrow transplant recipients with pneumonia. The diagnostic sensitivity of immunofluorescence (IF) using this antibody exceeded that of standard histology performed on touch imprints and frozen and permanent lung sections and was equal to viral culture and in situ CMV nucleic acid hybridization. In comparison with patients with CMV pneumonia demonstrated histologically and by IF, those with negative histology and positive IF were more likely to have seroconverted before biopsy. Despite this evidence of an immune response to CMV pneumonia, the two groups did not differ in CMV positivity by culture or by hybridization, and their long-term survival was equally poor. The findings demonstrate that this antibody can play an important role in both the rapid diagnosis of CMV infection and the elucidation of CMV pathophysiology.