In two patients with a persisting, high antistreptolysin titre the antistreptolysin activity in both cases resided exclusively in a monoclonal IgG component in the serum. This component had all the characteristics of a true antibody. A history in both patients of arthritis with or without angina suggested that the monoclonal antibodies were reactive in origin, although definite proof was lacking. In one case there was a suggestion of incipient myeloma. Whenever an extremely high antistreptolysin titre persists after antibiotic treatment the possibility of paraproteinaemia should be considered.