Endoblast and hypoblast tissue, dissected from early chick embryos, was explanted and cultured on glass or plastic substrata. These tissues grew rapidly to form epithelial sheets. Under the same conditions, mesoderm, dissected without the aid of dissociating agents, grew poorly. After 24 h in culture, the mesoderm explants consisted of a sparse outgrowth of fibroblast-like cells. When pieces of mesoderm were seeded onto the dorsal surface of the epithelia, however, the cells penetrated the sheet and rapidly spread on the substratum within 4 h. If the epithelial sheet was detached from the substratum and the mesoderm then seeded onto areas of substratum previously occupied by epithelium, similar rapid spreading occurred. This effect could be produced in the absence of serum. The method used to remove the epithelium (EDTA, detergent or manual dissection) did not influence the result. When the substratum-attached material (SAM) was examined by scanning electron microscopy, 2 types of material were seen. One type appeared to be the remains of detached filopodia and cytoplasmic lamellae, while the other appeared to be of extracellular origin. Both these types reacted positively by immunofluorescence using anti-fibronectin serum. SAM derived from mesoderm reacted negatively. When mesoderm was cultured in the presence of plasma fibronectin on unmodified plastic or glass, spreading was complete in 4-5 h and thus was similar to mesoderm seeded onto SAM. The morphology of mesoderm explants on SAM or in the presence of plasma fibronectin was more epithelial than on untreated substratum in normal medium. Hypoblast and endoblast cultured in the presence of anti-fibronectin serum failed to spread normally, apparently being unable to attach to the substratum. Mesoderm did not spread rapidly on SAM in the presence of this antiserum. Cycloheximide reversibly inhibited the spreading of hypoblast and endoblast, and this effect could be eliminated, at least for hypoblast, by the addition of plasma fibronectin. Covering attachment sites on the substratum with bovine serum albumin, thereby preventing the attachment of SAM or fibronectin, also inhibited spreading. It is proposed that mesoderm cells have low levels of surface fibronectin in comparison with endoblast and hypoblast, and that this results in a comparatively low adhesiveness, which is important for its morphogenetic activity within the embryo.