Abstract Transforming growth factor-β (TGF-β) is a potent anti-inflammatory cytokine. Although this cytokine inhibits lipopolysaccharide (LPS)-mediated septic shock, the molecular mechanism of TGF-β is not well known. Since recent studies showed that c-Jun N-terminal kinase (JNK), one of the mitogen-activated protein kinases, plays an important role in LPS signalling, we focused here on the inhibitory action of TGF-β1 on LPS-stimulated JNK activity in mouse macrophages. TGF-β1 inhibited LPS-stimulation of phosphorylated JNK1 and JNK2 and consequently of JNK activity in the cells. This JNK activity resulted in a decreased level of phosphorylated c-Jun protein. Using Western blotting, we also observed TGF-β1 inhibition of newly synthesized c-Jun protein in LPS-stimulated cells. These results demonstrate that TGF-β1 inhibits LPS-stimulated JNK activity in mouse macrophages. Also, our present study suggests a possible inhibitory mechanism of TGF-β in signalling of LPS-induced inflammatory responses.