Red cells sensitized with autoantibodies are able to adhere in vitro to autologous monocytes and monocytes from healthy individuals. A direct relationship between the degree of sensitization and the percentage of rosettes was not observed, while such a correlation was found if red cells sensitized with anti-Rh alloantibodies were used. Sometimes the adherence of red cell from AIHA was observed although the sensitization was weaker than that of the control erythrocytes sensitized with anti-CD serum which did adhere to monocytes. In patients with AIHA some relation was found between the adherence assay, haemolysis in vivo and treatment with prednisone.