Immunohistochemical techniques with monoclonal antibodies against cytomegalovirus (CMV) immediate early (IEA) and early antigens (EA), and in situ hybridisation, were used to detect CMV infection in routinely obtained, formaldehyde fixed and paraffin wax embedded tissues taken from bone marrow transplant patients, who had died form interstitial pneumonia. To improve the rates of detection of CMV-IEA and EA the wax embedded material was pretreated with 0.4% pepsin/HCl at 37 degrees C for 30 minutes. This pretreatment was also advantageous for in situ hybridisation. In the patients with histological evidence of CMV infection or positive viral culture from the lung tissue, or both, viral proteins and nucleic acids were detected in lung, as well as in other organs. Immunohistochemical techniques proved superior in heavily infected but necrotic tissues. In control patients (patients who had died from interstitial pneumonia without any evidence of CMV, or with no interstitial pneumonia at all) in situ hybridisation showed no positive signal, while immunohistochemical techniques showed only a few positive cells in lung tissue of one of nine patients. In addition to CMV-DNA analysis, formaldehyde-fixed, paraffin wax embedded tissue is amenable to immunohistochemical analysis with CMV monoclonal antibodies.