There is evidence that blockade of the reticuloendothelial system (RES) by sequestration of autologous red blood cells (RBC) leads to an elevation of platelet counts in immune thrombocytopenia. To substantiate this hypothesis, 10 Rh₀(D)-positive adult patients (9 female, 1 male) with chronic autoimmune thrombocytopenic purpura (ITP) (1 to 21 years duration) were treated with low doses of intravenous IgG-anti-Rh₀(D) (200 to 1,000 μg per dose; 300 to 3,600 μg per course; administration within 1 to 5 days). All patients improved clinically as indicated by cessation of bleeding. In eight out of ten patients there was a rise in platelet count. Platelet increments were excellent (>100×10⁹/l) in one, good (50−100×10⁹/l) in three, fair (20−50×10⁹/l) in two and low (10−20×10⁹/l) in two patients. Splenectomized patients (N=4) had a poorer response than non-splenectomized patients (N=6) with mean increments of 16×10⁹/l (range 5−43×10⁹/l) versus 60×10⁹/l (range 10−110×10⁹/l). The increase in platelet counts persisted for seven to over 150 days. Transient and slight signs of haemolysis developed in seven out of ten patients (haemoglobin remained stable; increase of lactate dehydrogenase (>250 IU/l) in four, decrease of haptoglobin (<60 mg/dl) in five patients). The direct antiglobulin test became positive in all cases due to IgG1 without complement fixation. We conclude that the interaction of antibody-coated RBC with macrophages (and, probably, other means of RBC alteration) is a feasible therapeutic approach in selected cases of ITP and related conditions.