The effects of treatment with alkylating agents and prednisone on the paraprotein level, the serum viscosity and the clinical symptoms have been studied in four cases of Waldenström's macroglobulinaemia and in one case of IgA myeloma. Although the treatment occasionally had some effect, no correlation was observed between the paraprotein level and the clinical condition in the four macroglobulinaemia cases. In the myeloma case there was some correlation, although osteolytic foci appeared at a time when the production of paraprotein was considerably suppressed. It appears that neither the paraprotein level nor the serum viscosity can be used with certainty as an indicator of the activity of the pathological process. It is suggested that the paraprotein may not always be the principal cause of the symptoms of Waldenström's macroglobulinaemia.