This study was aimed at developing a method for high-efficiency transient transfection of macrophages. Seven methods were evaluated for transient transfection of murine macrophage RAW 264.7 cells. The highest transfection efficiency was achieved with DEAE-dextran, although the proportion of cells expressing the reporter gene did not exceed 20%. It was subsequently found that the cytomegalovirus plasmid promoter in these cells becomes methylated. When cells were treated with the methylation inhibitor 5-azacytidine, methylation of the plasmid promoter was abolished and a dose-dependent stimulation of reporter gene expression was observed with expression achieved in more than 80% of cells. Treatment of cells with 5-azacytidine also caused increased efficiency of transfection of macrophages with plasmids driven by RSV, SV40, and EF-1alpha promoters and transient transfection of human HepG2 cells. Inhibition of methylation also increased the amount and activity of sterol 27-hydroxylase (CYP27A1) detected in RAW 264.7 cells transfected with a CYP27A1 expression plasmid. Treatment of cells with 5-azacytidine alone did not affect either cholesterol efflux from nontransfected cells or expression of ABCA1 and CYP27A1. However, transfection with CYP27A1 led to a 2- to 4-fold increase of cholesterol efflux. We conclude that treatment with 5-azacytidine can be used for high-efficiency transient transfection of macrophages.