Oligoclonal paraproteinaemia occurred in two patients with malignant lymphocytic disease (highly malignant non-Hodgkin lymphoma, B-cell type of acute lymphocytic leukaemia), in one case during a cytomegalovirus infection and in the other during an infectious mononucleosis. At that time both patients were in complete remission. Paraproteinaemia in the first case disappeared within a year where the transformation from an initially four-banded paraproteinaemia (2 IgM-lambda and 2 IgG-lambda) into a three-banded paraproteinaemia (IgG-lambda) could be observed. In the second patient the concentration of the paraprotein decreased considerably. Because both patients were no longer under cytostatic treatment after manifestation of the paraproteinaemia, and were in complete remission during the whole of the observation period (4 years and 1 year), a direct relationship with the primary disease is unlikely. The transitory paraproteinaemia appears to be rather the result of an acquired defect of the immune system.