Examination of the factors involved in primary mesodermal migration in the mouse has been complicated by the lack of a suitable in vitro model. We have developed a new culture system using primitive streak stage embryos denuded of primitive endoderm, which allows easy observation and manipulation of the outgrowing cells. The cells migrating away from these explants were shown by immunocytochemistry to express vimentin and an epitope of the I antigen recognised by the antibody C6, both of which are present on the newly emerged mesoderm and not on the embryonic ectoderm in sections of embryos in utero. Conversely, cytokeratin, stage-specific embryonic antigen 1 (SSEA-1), E-cadherin and desmoplakin are expressed by the embryonic ectoderm but lost during mesoderm formation in vivo. They are absent or expressed very weakly by the migrated cells in vitro. In addition, only explants of the ectoplacental cone (EPC) and visceral endoderm alone, expressed a carbohydrate epitope (recognised by monoclonal antibody BOO6), characteristic of the EPC and primitive endoderm in utero, but absent from mesoderm. Thus we conclude that the cells which outgrow in this system are indeed mesodermal in phenotype. We have confirmed the work of others in demonstrating the presence of fibronectin (FN) and laminin (LN) in the migratory path of the mesoderm, at the ectoderm-visceral endoderm interface. We also report that the β1 integrin subunit of the FN and LN receptor is expressed by mesodermal cells at this interface. Using our in vitro model we have examined the role of the extracellular matrix (ECM) in mesodermal migration. Mesodermal cells migrate further and faster on substrates coated with FN or LN, and this increased migration is abolished by appropriate blocking antibodies. We conclude that the ECM, in particular FN and LN, plays an important role in the migration of primary mesodermal cells during gastrulation in the mouse embryo.