Cell surface modification during mesenchyme ingression was examined using a monoclonal antibody (mAb), anti-Epith-1 mAb, raised against a protein (Epith-1) that was confined to the lateral surface of the epithelial cells in embryo of the sea urchin, Temnopleurus hardwicki. The mAb epitope was N-glycosylated oligosaccharides of 160 kDa monomeric Epith-1 protein. The glycoprotein was negatively charged, and its isoelectric point (IP) was 4.98. The mAb, however, is not immunologically cross-reactive with other sea urchin embryos including Hemicentrotus pulcherrimus, Strongylocentrotus nudus, and Scaphechinus mirabilis. Epith-1 is present initially in the cytoplasm of unfertilized eggs. Cytoplasmic Epith-1 shifted to the cell surface to be integrated in plasma membrane during the first cleavage, and remained there during early embryogenesis by retaining the same relative molecular mass (Mr). During primary and secondary mesenchyme ingression periods, however, Epith-1 disappears from the presumptive mesenchyme cell surface that was associated with internalization of the protein. In plutei, an additional anti-Epith-1 mAb-positive protein appears at the 142 kDa region, which was not associated with any visible alteration of the histologic localization of the protein in larvae. Anti-Epith-1 mAb IgG did not inhibit the reaggregation of epithelial cells in vitro, which suggests that either the protein is not involved in cell-cell adhesion or that the mAb is not recognizing the active site of the protein.