The distribution of fibronectin and laminin in early chick embryos has been studied by immunohistochemical method. Fibronectin was detected not only at the basement membranes of neural tube, dorsal aortae and somites but also at the entire mesenchyme area. But laminin only appeared at the basement membranes. During the development of chick embryo, the distribution pattern of these glycoproteins was not changed but showed strong intensity of staining. The dorsal aorta of a 2-day-old chick embryo had several layer of fibronectin-producing cells, but was devoid of laminin. Laminin expression commenced faintly after Day 3. In the brain capillary wall, fibronectin was distributed but laminin was not at Day 10. In the cardiac jelly, fibronectin was detected as thin dotted strands but laminin was not expressed at Day 2. Along the endocardium, fibronectin was detected but laminin was not found. After Day 3, fibronectin was found as increased in endocardium, myocardium and cardiac jelly, whereas laminin expression commenced. Cushion mesenchymal cell migration and development of myocardial trabeculae were inhibited by antifibronectin injected in the cardiac jelly but not anti-laminin. Conclusively, this study suggests that fibronectin and laminin are glycoproteins in the extracellular matrix but there is a difference between their distributions. Cushion mesenchymal cell migration from endocardium and development of trabeculae in chick embryonic heart are inhibited by anti-fibronectin. This result suggests that fibronectin is a `stepping stone` for the embryonic migrating cells and triggers cell migration. But laminin may not serve as a matrix for cell migration.