In patients with IgG incomplete non-complement binding warm autoantibodies, the subclass composition of the antibodies was studied in relation to the occurrence of increased haemolysis in vivo and the adherence of the patients red cells to peripheral blood monocytes (PBM) in vitro. The presence of IgG3 autoantibodies was almost always accompanied by haemolytic anaemia, but the presence of IgG1 autoantibodies only in some patients but not in others. IgG2 and IgG4 autoantibodies were not associated with increased red cell destruction. A relation identical to that between subclass composition and increased haemolysis was found between subclass composition and adherence of the patients erythrocytes to PBM and thus a strong correlation between positive adherence in vitro and increased red cell destruction in vivo. These results support an important role of adherence to mononuclear phagocytic cells in the destruction of red cells sensitized with non-complement binding IgG antibodies. Strong indications were found that IgG1 autoantibodies are of two kinds, only one of which causes adherence to phagocytes and thus increased red cell destruction.