Recent evidence suggests that bacterial DNA activates immune responses. Here we showed that TNF-alpha mRNA was induced in bone marrow-derived macrophages and the macrophage cell line RAW 264 by plasmid DNA, but not by DNaseI-digested plasmid, plasmid methylated on CpG dinucleotides, or by vertebrate genomic DNA, which is naturally largely methylated on these sequences. Synthetic polynucleotides poly d(I-C) and poly I x poly C also induced TNF-alpha. IL-1 beta and plasminogen activator inhibitor-2 mRNAs were induced by plasmid DNA, and IFN-gamma-pretreated macrophages responded to DNA with induction of inducible nitric oxide synthase. The HIV-1 long terminal repeat was activated by exogenous DNA in a manner similar to TNF-alpha, and was also activated by a CpG-containing oligonucleotide. Transcription factor nuclear factor-kappa B (NF-kappa B) is involved in regulation of the HIV-1 long terminal repeat and many inflammatory response genes. NF-kappa B binding activity was increased by plasmid DNA. An important question is whether these effects involve DNA binding to a cell surface receptor that signals to the interior, or whether internalization is necessary. Here we found that plasmid was taken up by RAW 264 cells and remained sufficiently intact to code for luciferase protein. Results suggest that DNA is taken up by macrophages and characteristic bacterial DNA sequences, which include an unmethylated CpG sequence, activate a signaling cascade leading to activation of NF-kappa B and inflammatory gene induction. Relevance to DNA vaccination, gene therapy, antisense, and transfection studies is discussed.