Members of the Rel family of proteins have been identified in Drosophila, an echinoderm, Xenopus, birds and mammals. Dimers of Rel proteins form the transcription factor nuclear factor κB (NF-κB) that rapidly activates genes encoding cytokines, cell surface receptors, cell adhesion molecules and acute phase proteins. Evidence suggests that xenobiotic compounds also may alter the activation of NF-κB. This study had a dual objective of identifying members of the Rel family and examining their activation by xenobiotic compounds in a marine fish model, scup ( Stenotomus chrysops). A DNA-protein crosslinking technique demonstrated that liver, kidney and heart each had at least three nuclear proteins that showed specific binding to an NF-κB consensus sequence, with molecular weights suggesting that the proteins potentially corresponded to mouse p50, p65 (RelA) and c-rel. In addition, an ≈35kD NF-κB binding protein was evident in liver and kidney. The 50 kD protein was immunoprecipitated by mammalian p50-specific antibodies. The presence of Rel members in fish implied by those results was confirmed by RT-PCR cloning of a Rel homology domain (an apparent c-rel) from scup liver. NF-κB activation occurred in vehicle-treated fish, but this appeared to decrease over time. In fish treated with 0.01 or 1 mg 3,3′,4,4′,5-pentachlorobiphenyl per kg, NF-κB activation in liver did not decrease, and there was a 6–8-fold increase in activation 16–18 days following treatment. Treatment with 10 mg benzo[a]pyrene/kg had no effect on NF-κB-DNA binding, either at 3 or 6 days following treatment. The data show that the Rel family of proteins is present in fish, represented at least by a p50/105 homologue, and support a hypothesis that some aryl hydrocarbon receptor agonists can activate NF-κB in vivo.