A modification of the Polybrene technique for red blood cell antibody characterization has been employed to differentiate the panhemagglutinins arising during methyldopa administration from those accompanying other disease states. Dissociation characteristics of methyldopa-associated antigen-antibody complexes were determined by temperature gradient dissociation technique. Data obtained by this technique for cell-bound antibody were found to distinguish this antibody from those accompanying systemic lupus erythematosus (SLE) and Pronestyl therapy. Graphic data derived from temperature gradient dissociation curves at varying antibody concentrations were obtained for the methyldopa-induced serum antibodies. Results obtained with samples from all six patients were found to be relatively uniform in relation to each other, and different from similarly derived results for red cell antibodies accompanying idiopathic autoimmune hemolytic anemia and Hodgkin’s disease. By means of these procedures, as well as standard blood banking techniques, distinguishing features are described that permit in vitro segregation of these distinct groups of red cell autoantibodies.