IFN-α2b inhibited proplatelet formation (PPF) and production of platelets from primary human megakaryocytes. Human bone marrow–derived CD34⁺ cells were cultured with 10 ng/mL TPO for 7 days. After collecting large megakaryocytes using velocity sedimentation, we maintained the cells with 10 ng/mL TPO in combination with 10 ng/mL IFN-α2b in 96-well plates for 6 days. (A) The numbers of megakaryocytes displaying PPF were counted under an inverted microscope at ×200. We counted 500 cells for each sample 3 times. Data are means plus or minus SEM (n = 3). (B) Platelets in the culture supernatant of primary human megakaryocytes were stained with antihuman CD41a and CD42a antibodies and counted by flow cytometry. Data are means plus or minus SEM (n = 3). (C) A representative picture of PPF with TPO and TPO plus IFN-α2b. Megakaryocytes were stained with Alexa Fluor 555-conjugated β-tubulin (9F3) antibody on day 12 and photographed under confocal laser microscopy. (D) IFN-α2b decreased the size of human megakaryocytes. Megakaryocytes were collected on day 13 and spun down on to glass slides. After staining with Wright-Giemsa solution, the diameters of the megakaryocytes were measured with a microscope using a scale within an eyepiece.