Tumor necrosis factor-α (TNF) exerts its transcriptional effects via activation of nuclear transcription factor-κ B (NF-κB). NF-κB is sequestered in the cytosol by IκBα and, in most cells, released upon serine phosphorylation of this inhibitory protein which then undergoes rapid, ubiquitin-dependent degradation. In contrast, we find TNF induction of NF- κB in murine bone marrow macrophages (BMMs), is mediated, by c-Src, in a cell, and cytokine specific manner. The non-receptor tyrosine kinase is rapidly mobilized and activated upon TNF exposure. Within the same time frame, TNF induced c-Src associates with IκBα in a long lived complex. The proto-oncogene, when associated with IκBα phosphorylates the inhibitory protein on tyrosine 42. Consistent with the pivotal role played by c-Src in TNF-induced IκBα tyrosine phosphorylation, NF-κB activation, by the cytokine, is markedly delayed and reduced in c-src-/BMMs. Underscoring the physiological significance of c-Src activation of NF-κB, TNF induction of IL-6, which is an NF-κB mediated event, is substantially diminished in c- src-/- BMMs.