Macrophages are putative target cells for expressing an exogenous gene with therapeutical effects. Knowing that macrophages express membrane lectins mediating endocytosis of their ligands, DNA/glycosylated polylysine complexes were used to transfect human blood monocyte-derived macrophages. Monocytes from human peripheral blood were matured in culture for 7 days to differentiate into macrophage-like cells in the presence of granulocyte-macrophage colony-stimulating factor (GM-CSF). Adherent cells, which displayed characteristic macrophage markers, CD 14, CD 11b, HLA-DR, and HLA-ABC antigens and mannose receptor, were transfected by DNA/glycosylated polylysine complexes in the presence of chloroquine. The luciferase reporter gene expression was maximal 24 hr after transfection with a DNA/mannosylated polylysine complex and by using plasmids in which the promoters (either the long terminal repeat of the human immunodeficiency virus or the human cytomegalovirus) drove the luciferase gene expression. Luciferase gene expression was lower when the promoter was the early region of the large T antigen of SV40 virus. Transfection mediated by DNA/mannosylated polylysine complexes was much more efficient than with DEAE-dextran or lipofectin. The possibility of transferring and expressing an exogenous gene into macrophage-like cells by using a nonimmunogenic synthetic vector as a DNA carrier opens new ways to develop nonviral gene therapy strategies.