As a basis towards a better understanding of the role of the pregnancy-specific glycoprotein (PSG) family in the maintenance of pregnancy, detailed investigations are described on the expression of a recently identified rat PSG gene (rnCGMl) at the mRNA and protein levels. Using specific oligonucleotide primers, rnCGMl transcripts were identified after reverse transcription, polymerase chain reaction, and hybridization with a radiolabelled, internal oligonucleotide. Transcripts were only found in significant amounts in placenta. In situ hybridization visualized rnCGM1 transcripts at day 14 post coitum (P.c.), in secondary trophoblast giant cells and in the spongiotrophoblast. Only those secondary giant cells lining the maternal decidua were positive. In contrast, primary giant cells did not contain rnCGMl mRNA. At day 18 P.c., rnCGMl transcripts were almost exclusively detectable in the spongiotrophoblast. No rnCGMl transcripts were found in rat embryos of these two developmental stages. Rabbit antisera were generated against the amino-terminal immunoglobulin variable-like domain and against a synthetic peptide containing the last 13 carboxy-terminal amino acids of rnCGMI. Both antisera recognized a 124 kDa protein in day 18 rat placental extracts as identified by Western blot analysis. The anti-peptide antiserum recognized a 116 kDa protein in the serum of a 14 day p.c. pregnant rat that is absent from the sera of non-pregnant females. Taken together, these results confirm exclusive expression of rnCGMl in the rat trophoblast, but unlike human PSG, negligible or no expression is found in other organs, such as fetal liver or salivary glands, indicating a more specialized function of rnCGM1. Its spatiotemporal expression pattern is conducive with a potential role of PSG in protecting the fetus against the maternal immune system andlor in regulating the invasive growth of trophoblast cells. 0 1993 Wiley-Liss, Inc.