We studied de novo protein biosynthesis in platelets of normal adult donors and in newly formed platelets isolated from splenectomized patients with idiopathic thrombocytopenic purpura (ITP). After metabolic labelling of platelet proteins, performed with different radiolabelled amino acids or carbohydrates, a tenfold increase in incorporation of radioactivity into trichloroacetic‐acid‐precipitable material was obtained with ITP platelets compared to control platelets. Electron microscopic studies of ITP platelets revealed the presence of rough endoplasmic reticulum and polyribosomes, providing morphological evidence for protein synthesis. SDS‐PAGE of radiolabelled ITP platelet proteins followed by autoradiography showed that [³⁵S]methionine and [³H]leucine were incorporated into almost all Coomassie‐blue‐stained proteins whereas [³H]carbohydrates only labelled a few bands. Using crossed‐immunoelectrophoresis we identified some of the labelled platelet compounds and demonstrated that major membrane glycoproteins (GPIb, IIb, IIIa) and alpha‐granule proteins, such as fibrinogen, thrombospondin, albumin and von Willebrand factor, were synthesized in newly formed circulating platelets.