Abstract.11 patients with anaemia and thrombocytopenia due to bone marrow failure each received 2 sets of red‐cell transfusions. They were randomised to receive 1 blood transfusion through a standard 170‐μm giving set filter and another through a 40 μm micro‐aggregate filter. After transfusion, the mean fall in platelet count was 15times 109/1 (41.7%) and 1.4times 109/1 (4.6%), respectively (p<0.01). The results confirm the findings of a previous study which showed that micro‐aggregate filters prevent a post‐transfusional decrease in platelet counts. In addition, this new study shows that this is clinically relevant in patients who are thrombocytopenic at the time of their blood