In the presence of thrombopoietin (TPO), megakaryocytes mature by polyploidization and cytoplasmic maturation, and the matured megakaryocytes induce drastic morphological change and proplatelet formation and release a number of platelets. However, the regulatory mechanism of this unique differentiation process is still obscure. We therefore attempted to identify the factors, expression of which is induced by TPO stimulation in mouse bone marrow megakaryocytes. We isolated the mouse homolog of the neutrophil activating peptide-2 (NAP-2). Mouse NAP-2 cDNA encodes a predicted sequence of 113 amino acids and contains the Cys motif (CXC) found in other members of the alpha-chemokine family. At the amino acid level, the predicted mouse NAP-2 has 50.4%, 51.8%, and 72.6% identity with the predicted human, pig, and rat NAP-2, respectively. Northern blot analysis demonstrates that mouse NAP-2 is expressed only in spleen. Furthermore, the RT-PCR technique shows that the mouse NAP-2 gene is clearly upregulated by TPO stimulation in mouse megakaryocytes.