We identify the α₄subunit of integrin as a predominant integrin expressed by neural crest cells in both avian and murine embryos. Using degenerate primers, we obtained a PCR fragment of the chick integrin α₄subunit that was subsequently used to clone the full-length subunit with a predicted amino acid sequence 60% identical to human and mouse α₄subunits.In situhybridization demonstrates that chick integrin α₄mRNA is expressed at high levels by migrating neural crest cells and neural crest-derived ganglia at both cranial and trunk levels. An antibody against the murine α₄subunit revealed similar distribution patterns in mouse to chick. In addition to neural crest cells, the integrin α₄subunit was later observed on the muscle masses of the limb, the apical ectodermal ridge, and the developing liver. To examine the functional role of the integrin α₄subunit in neural crest cell migration, we used an explant preparation that allows visualization of neural crest cells in their normal environment with or without perturbing reagents. In the presence of a blocking antibody against the mouse integrin α₄subunit, there was a profound abrogation of neural crest cell migration at trunk and hindbrain levels. Both the numbers of migrating neural crest cells and the total distance traversed were markedly reduced. Similarly, avian embryos injected with synthetic peptides that contain the integrin α₄binding site in fibronectin displayed abnormal neural crest cell migration. Our results suggest that the integrin α₄subunit is important for normal neural crest cell migration and may be one of the primary α subunits used for neural crest cell migrationin vivo.Furthermore, the integrin α₄subunit represents a useful neural crest marker in the mouse.