The complex host-pathogen interplay involves the recognition of the pathogen by the host's innate immune system and countermeasures taken by the pathogen. Detection of invading bacteria by the host leads to rapid activation of the transcription factor NF-kappaB, followed by inflammation and eradication of the intruders. In response, some pathogens, including enteropathogenic Escherichia coli (EPEC), acquired means of blocking NF-kappaB activation. We show that inhibition of NF-kappaB activation by EPEC involves the injection of NleE into the host cell. Importantly, we show that NleE inhibits NF-kappaB activation by preventing activation of IKKbeta and consequently the degradation of the NF-kappaB inhibitor, IkappaB. This NleE activity is enhanced by, but is not dependent on, a second injected effector, NleB. In conclusion, this study describes two effectors, NleB and NleE, with no similarity to other known proteins, used by pathogens to manipulate NF-kappaB signaling pathways.