We have examined the expression of the vav protooncogene during mouse embryogenesis using RNase protection assays, in situ hybridization, and immunocytochemical analysis. vav gene transcripts were first detected in E11. 5 embryos in the blood-forming islands and megakaryocytes of the fetal liver. During diversification of hematopoietic activity in the embryo, vav gene expression became down-regulated in the liver and activated in thymus and spleen. In newborn animals, vav expression was also confined to hematopoietic tissues, with the exception of the ameloblastic cell layer at the latest stages of tooth morphogenesis. In the adult, vav transcripts were found in spleen, thymus, lymph nodes, and bone marrow, but not in liver. In spleen, vav transcripts were concentrated in the white pulp areas, whereas in the red pulp, the vav transcripts appeared to be primarily localized in the megakaryocytes. In thymus, vav expression was found to be more abundant in the cortical areas than in the medulla. In agreement with these observations, purified thymic lymphocytes showed heterogeneous immunoreactivity against the Vav protein, whereas splenic lymphocytes and bone marrow-derived cells displayed rather uniform levels of expression. These observations suggest that the vav protooncogene plays an important role in the signal transduction pathways that regulate the development and maintenance of the hematopoietic system.