1. Using the RT/PCR method, we examined mRNA expression of several inflammatory factors in mouse embryos during mid-late embryonal development. mRNAs of tumor necrosis factor (TNF)-alpha, TNF-beta, their receptors (TNF-RI, TNF-RII), transforming growth factor (TGF)-beta, were expressed constitutively in most of the embryonic tissues. 2. While mRNAs of other factors, interleukin (IL)-1 alpha, IL-1 beta, IL-3, IL-6, granurocyte-colony stimulating factor (G-CSF), leukaemia inhibitory factor (LIF), and interferon (IFN)-gamma were only limitedly expressed. 3. The mRNAs of several complement components (C2, C3, C4, C5) and receptors (CR1, CR2) were also detected. Among them, the expression of C3 and CR1 were prominent. These results strongly support our idea that inflammation-like system play an important role to regulate embryogenesis.