The use of the Cre-loxP recombination system allows the conditional inactivation of genes in mice. The availability of transgenic mice in which the Cre recombinase expression is highly cell type specific is a prerequisite to successfully use this system. We previously have characterized regulatory regions of the mouse flk-1 gene sufficient for endothelial cell-specific expression of the LacZ reporter gene in transgenic mice. These regions were fused to the Cre recombinase gene, and transgenic mouse lines were generated. In the resulting flk-1-Cre transgenic mice, specificity of Cre activity was determined by cross-breeding with the reporter mouse lines Rosa26R or CAG-CAT-LacZ. We examined double-transgenic mice at different stages of embryonic development (E9.5-E16.5) and organs of adult animals by LacZ staining. Strong endothelium-specific staining of most vascular beds was observed in embryos older than E11.5 in one or E13.5 in a second line. In addition, the neovasculature of experimental BFS-1 tumors expressed the transgene. These lines will be valuable for the conditional inactivation of floxed target genes in endothelial cells of the embryonic vascular system.