We report a case of marked cytomegalovirus (CMV)-antigenemia determined by direct immunoperoxidase staining using a peroxidase-labeled monoclonal antibody HRP-C7, after conventional chemotherapy for malignant lymphoma. A 65-year-old Japanese man suffered from unexplained fever, mild liver dysfunction and an abnormal shadow in the lung after hematopoietic recovery from intensive chemotherapy for T-cell non-Hodgkin's lymphoma in the leukemic phase. The assay for CMV-antigenemia revealed that he had an active CMV infection. After treatment with ganciclovir and gamma-globulin, his symptoms and signs improved with the decrease of CMV antigen-positive leukocytes. CMV disease should be considered in these situations, and the CMV antigen-detection assay is useful for rapid diagnosis of CMV infection.