The aim of this study was to determine the extent of human endogenous retrovirus (HERV) gene translation in megakaryocytes cultured from peripheral blood stem cells of patients with essential thrombocythemia previously reported with platelet-associated HERV sequences and reverse transcriptase activity. Terminally differentiated megakaryocytes derived from circulating stem cells in serum-free medium supplemented with stem cell factor and thrombopoietin were processed for electron microscopic immunostaining using a monoclonal antibody against the gag protein of HERV-K10 and an electron dense gold-labeled secondary antibody. We found that HERV-K gag protein was detected as clusters in the cytoplasm as well as associated with viral particles budding from the cell membrane and into intracellular vacuoles in megakaryocytes from two patients with essential thrombocythemia. None of these structures was observed in megakaryocytes from a normal control or from a patient with chronic myelocytic leukemia. This is the first evidence of HERV-K protein synthesis (gene translation) in human tissue other than seminomas, placenta, or fetal tissue. Translation of the HERV-K gag gene with subsequent packaging of the protein product into viral particles adds a new and important dimension to future studies on the role of HERVs in the myeloproliferative diseases.