Tissue-plasminogen activator (t-PA) has been identified in human megakaryocytes isolated from human rib fragments or from culture cells. The presence of t-PA antigen in the endoplasmic reticulum of mature cells was demonstrated by ultrastructural immunochemistry. These results suggest that t-PA can be synthesized by megakaryocytes as well as by vascular endothelial cells. In order to confirm this possibility, in situ hybridization using antisense RNA biotinylated probe was used in megakaryocyte culture at different steps of maturation. The presence of t-PA-RNA was clearly demonstrated at an early step of differentiation. Biotin staining was enhanced in mature cells and this was correlated with the detection of t-PA antigen by immunocytochemistry.