Nuclear factor ${kappa}B$ (NF-${kappa}B$) is a nuclear transcription factor that modulates the expression of inflammatory cytokines such as tumor necrosis factor (TNF)-${alpha}$. trans-10, cis-12 (t10c12)-conjugated linoleic acid (CLA) participates in the inhibition of TNF-${alpha}$ production upon lipopolysaccharide (LPS)-stimulation. However, in our previous study, t10c12-CLA enhanced the production of TNF-${alpha}$ by LPS-unstimulated porcine peripheral blood mononuclear cells (PBMCs) and RAW 264.7 macrophages in vitro. To resolve this apparent contradiction, we hypothesized that the effect of t10c12-CLA on TNF-${alpha}$ production depends on NF-${kappa}B$ activation induced by LPS stimulation. To test this hypothesis, we assessed the in vitro effect of t10c12-CLA on TNF-${alpha}$ production and NF-${kappa}B$ p65 activity in LPS-stimulated and LPS-unstimulated porcine PBMCs. t10c12-CLA treatment resulted in increased TNF-${alpha}$ production by LPS-unstimulated PBMCs but decreased TNF-${alpha}$ production by LPS-stimulated PBMCs. t10c12-CLA increased the degradation of inhibitory ${kappa}B$ ($I{kappa}B$)-${alpha}$ protein and activated NF-${kappa}B$ p65 in LPS-unstimulated PBMCs, but had the opposite effect in LPS-stimulated PBMCs. Notably, t10c12-CLA enhanced NF-${kappa}B$ p65 binding activity in LPS-unstimulated PBMCs exposed to caffeic acid phenethyl ester (CAPE), a NF-${kappa}B$ inhibitor. Conversely, it inhibited NF-${kappa}B$ p65 binding activity in LPS-stimulated PBMCs exposed to CAPE. These results suggest that t10c12-CLA may have different actions under different physiological conditions, and that its effect may be associated with a change in NF-${kappa}B$ p65 activity.