Embryonic stem cells (ESCs) can be propagated in vitro on feeder layers of mouse STO fibroblast cells. The STO cells secrete several cytokines that are essential for ESCs to maintain their undifferentiated state. In this study, we found significant growth inhibition of mouse ESCs (mESCs) cultured on STO cells infected with adenovirus containing a dominant-negative mutant form of $I{kappa}B$ (rAd-$dnI{kappa}B$). This blockage of the NF-${kappa}B$ signal pathway in STO cells led to a significant decrease in [$^3H$]thymidine incorporation and colony formation of mESCs. Expression profile of cytokines secreted from the STO cells revealed an increase in the bone morphogenetic protein4 (BMP4) transcript level in the STO cells infected with adenoviral vector encoding dominant negative $I{kappa}B$ (rAd-$dnI{kappa}B$). These results suggested that the NF-${kappa}B$ signaling pathway represses expression of BMP4 in STO feeder cells. Conditioned medium from the rAd-$dnI{kappa}B$-infected STO cells also significantly reduced the colony size of mESCs. Addition of BMP4 prevented colony formation of mESCs cultured in the conditioned medium. Our finding suggested that an excess of BMP4 in the conditioned medium also inhibits proliferation of mESCs.