Liver X Receptors (LXRs) alpha and beta are oxysterol-activated nuclear receptors involved in the control of lipid metabolism and inflammation. Pharmacological activation of LXR is promising in the treatment of atherosclerosis since it can promote cholesterol efflux from macrophages and prevent foam cell formation. However, the development of LXR agonists has been limited by undesirable side-effects such as hepatic steatosis mediated by LXRalpha activation. Therefore, it has been proposed that targeting LXRalpha activators to extrahepatic tissues or using LXRbeta-specific activators could be used as alternative strategies. It is not clear whether these molecules will retain the full atheroprotective potential of non-selective agonists. Our aim was therefore to determine the contribution of LXRalpha and LXRbeta to the control of cholesterol efflux in human macrophages. LXRalpha and/or LXRbeta expression was suppressed by small interfering RNAs in human primary macrophages treated or not with synthetic LXRalpha/beta dual agonists T0901317 and GW3965. We observed that LXRbeta silencing had no detectable impact on the expression of LXR-target genes such as ABCA1 and ABCG1. Moreover it did not affect cholesterol efflux. In contrast, LXRalpha silencing reduced the response of these LXR-target genes to LXR agonist and inhibited cholesterol efflux to ApoA-I, HDL(2) or to endogenous ApoE. Importantly, no differences were observed between LXRalpha and LXRalpha/beta knockdown conditions. Altogether, our data demonstrate that LXRbeta activation is unable to maintain maximal cholesterol efflux capacities in human primary macrophages when LXRalpha expression is impaired. In contrast to earlier mouse studies, LXRalpha levels appear as a limiting factor for macrophage cholesterol efflux in humans.