Conjugated linoleic acid (CLA) isomers are dietary fatty acids that modulate gene expression in many cell types. We have previously reported that specifically trans-9,trans-11 (t9,t11)-CLA induces expression of genes involved in lipid metabolism of human macrophages. To elucidate the molecular mechanism underlying this transcriptional activation, we asked whether t9,t11-CLA affects activity of liver X receptor (LXR) alpha, a major regulator of macrophage lipid metabolism. Here we show that t9,t11-CLA is a regulator of LXRalpha. We further demonstrate that the CLA isomer induces expression of direct LXRalpha target genes in human primary macrophages. Knockdown of LXRalpha with RNA interference in THP-1 cells inhibited t9,t11-CLA mediated activation of LXRalpha including its target genes. To evaluate the effective concentration range of t9,t11-CLA, human primary macrophages were treated with various doses of CLA and well known natural and synthetic LXR agonists and mRNA expression of ABCA1 and ABCG1 was analyzed. Incubation of human macrophages with 10 microM t9,t11-CLA led to a significant modulation of ABCA1 and ABCG1 transcription and caused enhanced cholesterol efflux to high density lipoproteins and apolipoprotein AI. In summary, these data show that t9,t11-CLA is an agonist of LXRalpha in human macrophages and that its effects on macrophage lipid metabolism can be attributed to transcriptional regulations associated with this nuclear receptor.