An N-linked carbohydrate-containing extracellular matrix determinant plays a key role in sea urchin gastrulation. During the process of gastrulation in the sea urchin embryo, the vegetal plate invaginates to form the archenteron, a long narrow tube that extends across the blastocoel. Rearrangement of cells within the archenteron is thought to be a key component of the process of archenteron elongation. While these cell rearrangements have been well described, the mechanism of the rearrangements and the coordination of the movements of individual cells that results in the elongation of the archenteron are not well understood. We have identified a monoclonal antibody, called ECM 1, that recognizes an N-linked carbohydrate-containing epitope on several high-molecular-weight basal lamina glycoproteins. In an attempt to block the function of this determinant in development, the ECM 1 antibody was injected into the blastocoel of living embryos and the effects on morphogenesis were determined. Injection of intact ECM 1 IgG or monovalent Fab fragments blocks cell rearrangements during secondary invagination and cell movements during segmentation of the gut. Glycopeptides derived from the glycoproteins recognized by ECM 1 also inhibit cell rearrangements during secondary invagination when injected into the blastocoel. The inhibitory activity of these peptides is eliminated by digestion with N-glycosidase F or pronase, enzymes that also disrupt the ECM 1 determinant. ECM 1 recognizes a nonuniformly distributed determinant in the basal lamina and blastocoel matrix. This determinant is stored in cytoplasmic granules in the unfertilized egg and is deposited into the basal lamina by the blastula stage. The determinant becomes concentrated in the basal lamina in the vegetal region of the embryo early in gastrulation. At the prism stage, the determinant accumulates in the basal lamina and blastocoel matrix in the ventral region of the embryo. These data indicate that the vegetally localized, N-linked carbohydrate-containing determinant recognized by ECM 1 plays an important role in cell movements during archenteron morphogenesis in the sea urchin embryo.