Osteoblasts play a pivotal role in bone remodeling. The alkaline phosphatase (ALPase) activity was decreased in ROS 17/2.8 osteoblast treated with TNF-alpha (2, 5 or 10 ng/ml). The treatment of TNF-alpha inhibited osteoblast differentiation such as ALPase activity in ROS 17/2.8 osteoblast. TNF-gamma (10 ng/ml) increased NF-kappaB DNA binding activity in nuclear extracts of osteoblasts. The addition of NAC (N-acetyl cysteine), free radical scavenger, completely prevented TNF-alpha-induced activation of NF-kappaB. In addition, IkappaB alpha and IkappaB beta were rapidly degraded, allowing the activated NF-kappaB to enter the nucleus and promote gene transcription. To determine whether IkappaB alpha signal transduction pathway is important in the differentiation, we generated IkappaB (KD)-stably transfected ROS 17/2.8 cells. These IkappaB (KD) transfectants did not show any regulation of ALPase in osteoblasts. Here, we suggest that the degradations of IkappaB alpha and IkappaB beta and the following activation of NF-kappaB are the targets of NAC and that NF-kappaB transcription factor is a pivotal clue to regulation of differentiation in TNFalpha-exposed osteoblasts.