The nuclear factor kappa B, NF-kappa B, is activated by numerous inflammatory mediators, regulating biological responses that in part are dependent on the extracellular matrix. Analyses of the influence of cell-matrix interaction on NF-kappa B activation revealed that attachment to fibronectin caused an increase in NF-kappa B activity corresponding to 3-fold the level in cells plated on bare plastic. This effect was dependent on the degree of attachment and appeared to involve both the RGD motif of the cell-binding domain, and the heparin-binding domain of fibronectin. Fibronectin attachment specifically caused activation of the p50/p65 heterodimeric form of NF-kappa B, resulting in an increase of this complex to a level similar to that induced by cytokine stimulation. These data show that cell attachment has a pronounced influence on the level of NF-kappa B activity, affecting specific dimeric complexes, and suggest this to be a pathway through which effects depending on cell-matrix interaction can regulate cell behavior and cytokine responses.