We generated double-transgenic mice carrying cointegrated tissue-specific Gal4 and Gal4 reporter transgenes to direct transgene overexpression in the mononuclear phagocyte system (MPS). A modified promoter of the Csf1r (c-fms) gene, containing a deletion of the trophoblast-specific promoter, was used to drive the expression of Gal4VP16 transcriptional activator specifically in macrophages. This module was cointegrated with a fluorescent reporter, enhanced cyan fluorescent protein (ECFP), driven by a Gal4-dependent promoter. ECFP fluorescence was first detected in forming blood islands of the yolk sac at 8 dpc, then in macrophages in the yolk sac and the embryo proper. In adult mice ECFP was detected primarily in monocytes, tissue macrophages, microglia, and dendritic cells, including Langerhans cells of the skin. Crossing of these mice to transgenics containing tagged protein under control of a Gal4-dependent promoter directed expression of that protein in mononuclear phagocytes of double-transgenic animals. The new mouse line provides a useful tool for overexpression of transgenes in cells of the myeloid lineage, while simultaneously labeling them by ECFP expression.