The alpha 5 beta 1 integrin mediates cell adhesion and migration on fibronectin, a glycoprotein critical for normal vertebrate embryonic development. Indirect evidence reported to date suggests that this receptor also functions in the deposition of fibronectin matrices. Specificity of I7661-35Q was verified by its ability to immunoprecipitate a5b1 heterodimers from 125-I-surface labeled cells, reciprocal depletion of VLA-5 antigen from cell lysate with antiserum against the cytoplasmic domain of a5 integrin subunit, and immunoprecipitation of VLA-5 integrin from cells known to express this integrin. Cell lines tested to express a5b1, include: P19 cells, mouse melanoma B16F1 cells, mouse squamous carcoma SCC cells and thymocytes.