We report the case of a 70-year-old woman suffering from small lymphocytic, plasmocytoid lymphoma with abdominal lymphomas and infiltration of the lung and the bone marrow. A three-banded, IgGλ, IgMλ and IgAλ-paraproteinemia was determined using immunofixation. Because of the patient's high antibody titre against cytomegalovirus (CMV), the possible reactivity of these paraproteins with CMV was studied. The immunoglobulins were transferred to nitrocellulose sheets by a contact diffusion blotting system. CMV was applied to these sheets and the IgGλ-paraprotein was shown to bind CMV. The reactivity of only one of the paraproteins with CMV suggests an oligoclonal origin of this gammopathy. In addition to the malignant disease an abnormal immune response to a CMV infection could be the cause of this three-banded gammopathy.