CD47 deficiency results in lethal autoimmune haemolytic anaemia (AIHA) and mild spontaneous thrombocytopenia in non-obese diabetic mice. It is unknown whether CD47 has an impact on AIHA of the warm type or autoimmune thrombocytopenia (ITP) in humans. Healthy blood donors (n= 11), patients with AIHA (n= 13), patients with ITP (n= 18) and one patient with Rh(null) phenotype were investigated. CD47 expression on red blood cells (RBC), platelets, granulocytes and lymphocytes and in plasma was determined by quantitative flow cytometry. All types of blood cells studied were found to carry CD47. Although CD47 expression on Rh(null) RBCs was decreased, there was no significant difference between CD47 expression on RBCs of healthy blood donors and on those of patients with AIHA or ITP. Similarly, CD47 was detectable in the plasma of the studied subjects. No evidence for a pathogenetic role of CD47 in autoimmune haemolysis or thrombocytopenia in humans could be demonstrated.