Thrombopoietin (TPO) is a growth and differentiation factor for megakaryocyte-lineage cells. The receptor for TPO, c-MPL, is a member of the hematopoietic cytokine receptor family and has previously been shown to rapidly activate one or more cytoplasmic tyrosine kinases after ligand binding. In this study, we found that activation of the TPO receptor rapidly induced tyrosine phosphorylation of two members of the Jak tyrosine kinase family, JAK2 and TYK2, but not JAK1 or JAK3, in two different factor-dependent hematopoietic cell lines. The activation of both JAK2 and TYK2 was dose-and time-dependent and was associated with rapid tyrosine phosphorylation of a series of STAT proteins including STAT1, STAT3, and STAT5. Gel-shift assays indicated that one or more of these STATs is likely to participate in the formation of specific DNA-binding complexes. The activation of tyrosine kinases and signal propagation through tyrosine phosphorylation are likely to represent important initial steps in mediating the activities of TPO in myeloid cells.