In this report we identify a 130-kDa protein encoded by a sea urchin primary mesenchyme-specific cDNA clone, 18C6. The cDNA clone has been partially sequenced, and an open reading frame has been identified. A portion of this open reading frame has been expressed as a beta-galactosidase fusion protein in Escherichia coli, and antibodies to the fusion protein have been generated. These antibodies recognize a 130-kDa protein localized at the surface of primary mesenchyme cells and designated msp130. This is demonstrated to be the same 130-kDa protein recognized by the primary mesenchyme-specific monoclonal antibody B2C2, which recognizes a post-translational modification of the protein. RNA gel blots show that the transcript encoding msp130 is undetectable in egg RNA or 16-cell RNA but can be first detected in premesenchyme blastula embryos. The transcript accumulates significantly after primary mesenchyme cell ingression. Analysis of the expression of msp130 by indirect immunofluorescence staining of embryos and by immunoblots using fusion protein antibodies shows that the msp130 protein is first detectable soon after primary mesenchyme cell ingression.