A cDNA clone, p2-4, was isolated from mouse teratocarcinoma-derived parietal endoderm-like cells and used to analyze expression of the corresponding transcript during mouse embryogenesis. Nucleotide-sequence analysis revealed extensive homology between this clone and SPARC/osteonectin cDNA cloned from mouse parietal endoderm and bovine bone cells. The SPARC/osteonectin transcript became more abundant when embryonal carcinoma (EC) cells differentiated into parietal endoderm-like cells. In embryos, the transcript began to appear in the embryo proper on day 11 and continued to be expressed throughout the gestation period. The transcript was also present in extraembryonic membranes and placenta from days 9 and 11 onward, respectively. Thus, expression of the transcript was regulated during differentiation of EC cells and during embryogenesis. In adult mice, several non-bone tissues, including testis, also expressed the transcript. Analysis of germ-cell-deficient mice indicated that non-germ-cell components of the testis expressed the transcript. Analysis of mouse testicular cell lines further suggested that the transcript was abundant in Sertoli cells and Leydig cells. Cumulus oophorus cells that envelope the ovulated egg also expressed high levels of the transcript.