In order to explore the effect and mechanisms of tumor necrosis factor-alpha (TNF-alpha) on the activity of the acyl coenzyme A: cholesteryl acyltransferase (ACAT), THP-1 monocytes were cultured and induced to differentiate into macrophages with phorbol ester. TNF-alpha (60 ng/mL) was added at different time points into the macrophage-containing medium and the ACAT enzyme activity was measured by quantifying the incorporation of [1-(14)C] oleoyl CoA into cholesteryl esters. The expression of ACAT-1 protein and mRNA was respectively detected by Western blotting and RT-PCR in THP-1 macrophages 24 h after treatment with TNF-alpha (60 ng/mL). The results indicated that ACAT activity in THP-1 macrophages treated with TNF-alpha was increased in a time-dependent manner. The expression levels of ACAT-1 protein and mRNA were significantly increased in THP-1 macrophages after treatment with TNF-alpha (P<0.05). It was suggested that TNF-alpha could increase the activity of ACAT in THP-1 macrophages by up-regulating the expression of ACAT-1 gene.