Macrophages are crucial for the inflammatory response process because they release a number of proinflammatory mediators. 1,1,1 -Trichloro-2-(p-chlorophenyl)-2-(o-chlorophenyl)ethane (o,p'-DDT) has been reported to possess immunomodulatory properties activity. However, its influence on cytokine production or the functions of the macrophages remains unclear. This study investigated the effects of o,p'-DDT on the production of nitric oxide (NO) and proinflammatory cytokines (IL-1beta, IL-6, TNF-alpha) and analyzed the molecular mechanism in mouse macrophages. The addition of o,p'-DDT to macrophages induced NO and proinflammatory cytokines production in a dose-dependent manner. o,p'-DDT also increased inducible nitric oxide synthase (iNOS) and proinflammatory cytokines expression levels in the cells. NF-kappaB sites were identified in the promoter of the iNOS and proinflammatory cytokines genes. Pretreating the cells with NF-kappaB pathway inhibitors suppressed the iNOS and proinflammatory cytokines expression induced by o,p'-DDT. The transient expression and electrophoretic mobility shift assays with the NF-kappaB binding sites revealed that the o,p'-DDT-induced increase in the iNOS and proinflammatory cytokines expression level were mediated by the NF-kappaB transcription factor. However, pretreating the cells with o,p'-DDT and the NF-kappaB pathway inhibitors suppressed DDT-induced NF-kappaB activation. These results demonstrate that o,p'-DDT stimulates the production of NO and proinflammatory cytokines and can up-regulate the gene expression levels via NF-kappaB transactivation. Overall, the results of this study suggest evidence that o,p'-DDT might possess an inflammatory potential that is previously unrecognized immunomodulating activity of o,p'-DDT.