5 patients with autoimmune haemolytic anaemia (AIHA) of warm type (4 idiopathic, 1 associated with systemic lupus erythematosus and thrombocytopenia) were treated with high doses of i.v. immunoglobulin (IgG; Sandoglobulin; 2.0 g/kg body weight). IgG therapy was ineffective in all 5 cases as indicated by a lack of clinical improvement, continuous signs of accelerated red blood cell (RBC) destruction, and an unchanged survival rate of 51Cr-labelled autologous RBC in 4 patients studied. IgG infused at equivalent doses into 5 healthy volunteers led to an increase of IgG coating of autologous RBC (irrespective of the ABO blood groups) without concomitant changes of haemoglobin, haematocrit or reticulocytes, increase of serum IgM in 3/5, a decrease of serum C4 in 5/5, and a decrease of serum haptoglobin in 2/5 individuals. We conclude that IgG at a dose of 2.0 g/kg body weight is ineffective in AIHA. This may be caused by an increased, though clinically inapparent, interaction of IgG-coated autologous RBC with the mononuclear phagocyte system.