Procedures for diagnostics of cytomegalovirus infections include histopathology, cell culture, serology, and direct detection of viral antigens or nucleic acids within infected cells or tissues. In order to develop a new diagnostic reagent for viral antigen detection, we generated a mouse monoclonal antibody. This antibody was raised against a recombinant antigen representing part of the large phosphorylated structural protein pp150 of human cytomegalovirus. The monoclonal antibody was shown to be useful for antigen detection by immunofluorescence and immunoenzymatic staining in infected cells from cell culture as well as from infected organs. The antibody proved to be reactive even in paraffin-embedded sections from tissue specimens.