We have performed a systematic in vivo evaluation of gene expression for the glycoprotein (GP) Ibalpha subunit of the murine platelet adhesion receptor, GP Ib-IX-V. This study is warranted by in vitro observations of human GP Ibalpha expression in cells of nonhematopoietic lineage and reports of regulation of the GP Ibalpha gene by cytokines. However, an in vivo role for a GP Ib-IX-V receptor has not been established beyond that described for normal megakaryocyte/platelet physiology and hemostasis. Our Northern analysis of mouse organs showed high levels of GP Ibalpha mRNA in bone marrow with a similar expression pattern recapitulated in mice containing a luciferase transgene under the control of the murine GP Ibalpha promoter. Consistently high levels of luciferase activity were observed in the two hematopoietic organs of mice, bone marrow (1,400 relative light units/microg of protein [RLUs]) and spleen (500 RLUs). Reproducible, but low-levels of luciferase activity were observed in heart, aorta, and lung (30 to 60 RLUs). Among circulating blood cells, the luciferase activity was exclusively localized in platelets. No increase in GP Ibalpha mRNA or luciferase activity was observed after treatment of mice with lipopolysaccharides (LPS) or tumor necrosis factor-alpha (TNF-alpha). We conclude the murine GP Ibalpha promoter supports a high level of gene expression in megakaryocytes and can express heterologous proteins allowing an in vivo manipulation of platelet-specific proteins in the unique environment of a blood platelet.