Abstract: Expression of TGF-β mRNA and protein in mouse placenta was detected using in situ hybridization and immunohistochemistry.The results showed that TGF-β mRNA and protein were mainly detected in the deciduas between D7.5 and D9.5 of pregnancy and the expression gradually decreased.The data indicate that TGF-β might be involved in regulation of trophoblast invasion during this period and may act through inhibiting the activity of two types of extracellular proteinase,Pas and MMPs.On D10.5 of pregnancy,trophoblast giant cells expressed TGF-β mRNA and protein.This was consistent with the fact that the trophoblast giant cells stoped to proliferate and invade at this time.On D9.5,spongiotrophoblast cells began to express TGF-β mRNA,while its protein was detected in these cells on D10.5,and the expression of TGF-β mRNA reached the peak level between D11.5 and D12.5 indicating it maybe also involved in regulation of the formati on of fetal vasculature during this period.Furthermore,the maternal tolerance to fetal semi-allograft was partly attributed to the expression of TGF-β,because TGF-β could inhibit the maternal immune system.