Echinonectin (EN) is a dimeric galactosyl-binding protein found in sea urchin eggs and embryos. It had been postulated in earlier studies that EN is secreted into the hyaline layer, a stratified matrix deposited on the apical surface of cells, and serves as an attachment substrate for cells of the blastoderm. However, the dynamics of EN expression have rendered past observations difficult to interpret on this point and others. Radioiodination experiments in this study indicate that the bulk of EN is, at any one time, maintained in its vesicular compartment beneath the plasma membrane, but that a portion of the protein is secreted onto the cell surface during early development. The primary structure of EN was determined. The protein consists of a series of coagulation factor 5/8 repeats and discoidin-like lectin domains, and bears similarity to the secreted proteins DEL-1 and lactadherin from angiogenic endothelial cells. In situ hybridization analysis indicates that EN mRNA levels are regulated to coincide with periods of reduced motility in embryonic cells, supporting the postulate that the protein is involved in cell anchoring.