Two distinct cell types from the amphibian gastrula were compared with regard to their interactions in vitro with fibronectin (FN). Xenopus embryonic endoderm cells attach to FN substrates in a way characteristic of most cell types studied so far; that is, adhesion increases abruptly at a certain threshold concentration of FN, and maximal binding of cells already occurs at low FN concentrations (10 micrograms/ml). In contrast, embryonic ectodermal cells bind maximally to FN substrates only at unusually high concentrations of FN (200 micrograms/ml). This peculiar mode of attachment to FN has been characterized more closely. It is shown that the adhesion of ectodermal cells is modified by their interaction with a heparin-binding domain of the FN molecule. Furthermore, ectodermal cell adhesion increases very slowly with increasing FN concentrations. Despite these characteristic differences, both ectodermal and endodermal cells attach to the normal RGD cell-binding site of FN, as can be shown by competitive inhibition of adhesion by a hexapeptide containing the RGD sequence of amino acids.