Mice chimeric for the expression of α4 integrins were used to dissect the roles of these receptors in development and traffic of lymphoid and myeloid cells. During fetal life, T cell development is α4 independent, but after birth further production of T cells becomes α4 dependent. Precursors for both T and B cells require α4 integrins for normal development within the bone marrow. In contrast, monocytes and natural killer cells can develop normally without α4 integrins. Thus, there are lymphocyte-specific, developmentally regulated requirements for α4 integrins in hematopoiesis in the bone marrow. We also show that α4 integrins are essential for T cell homing to Peyer's patches, but not to other secondary lymphoid organs, including spleen, lymph nodes, and intestinal epithelium.