Paracentrotus lividus sea urchin nectin (Pl-nectin) is an extracellular matrix (ECM) protein of the sea urchin embryo on the apical surface of the ectoderm and has been shown to be an adhesive substrate for embryonic cells. A monoclonal antibody (McAb) to Pl-nectin was generated that inhibits the adhesion of blastula cells to Pl-nectin-coated substrates in an in vitro functional assay. To examine for possible in vivo functions of Pl-nectin, Fab fragments (Fabs) of Pl-nectin McAb were added to early blastulae. Ingression of primary mesenchyme cells was not affected by Fabs. As control embryos reached the pluteus stage, treated embryos showed a severe inhibition of skeletal elongation and patterning. When the Fabs were injected directly into the blastocoel, even at higher concentration than was applied externally, skeletogenesis was normal. Therefore, the effect of the antibody on spiculogenesis was indirect. The treatment was partially reversible as embryos eventually seemed to recover and elongate spicules, although with an incorrect patterning. Migration of pigment cells was also affected by the Fabs, since they did not disperse throughout the ectoderm but remained clustered in ectopic areas. In contrast, the development of endoderm structures was not affected. Our results indicate that in the sea urchin embryo the appropriate contact of ectodermal cells with outer ECM components is essential for the correct morphogenesis of inner mesodermal structures.