U937/RFP-luciferase cells are derived from the human pro-monocytic cell line U937 by stably integration of a constitutive RFP-firefly luciferase expression construct. U937 cells are able to differentiate into either dendritic cells or macrophages, depending on the surrounding signals. U937 cells are widely used for studies of monocyte/macrophage differentiation, the macrophage-tumor cell interaction and anti-cancer drug screening. U937/RFP-Luciferase cells stably express both RFP and firefly luciferase reporters, can be used for in vitro assays and in vivo imaging.