Hemato-oncological patients with chemotherapy-induced thrombocytopenia are a major recipient group of frequent platelet (PLT) transfusion. Prophylactic platelet transfusions are administered when platelet counts fall below 10 × 10⁹ PLT/L, to prevent severe or fatal bleeding. However, these prophylactic platelet transfusions do not always result in the prevention of bleeding. Pre- or post-transfusion acquired dysfunction of donor platelets in this respect could play a role. We previously reported intrinsic and transfusion-dependent platelet alterations in hemato-oncological patients. In particular, the expression of relevant platelet receptors was affected in donor platelets after incubation with patient’s plasma, which could explain, at least in part, the variable efficacy of platelet transfusions in these patients. In the present manuscript we show that plasma from acute myeloid leukemia (AML) patients undergoing chemotherapy inhibits functionality of allogenic platelets. Further proteomic analysis allowed us to observe alterations in the composition of plasma samples, and to identify key plasma components which could be responsible for platelet function inhibition and explain bleeding in patients notwithstanding platelet transfusions. We anticipate that with the obtained results, platelet transfusion support can be further personalized in patients receiving chemotherapy and applications might expand to maximize the clinical efficacy of procedures such as bone marrow transplantation.