The growth of endometrial cells in ectopic locations (endometriosis) is dependent on the establishment of an adequate blood supply. Neovascularization (angiogenesis) is therefore a vital step toward the progression of this disease. We first revealed the presence of a potent mitogenic activity for human endothelial cells in the culture medium of immortalized human endometriotic cells. The activity, measured by the level of [(3)H]-thymidine incorporation into the DNA of human coronary artery endothelial cells, was then purified by anion exchange high-performance liquid chromatography. Electrophoretic analysis of one of the bioactive fractions that markedly enhanced endothelial cell proliferation showed three distinct bands with apparent molecular masses of 15.8, 12.6, and 6.5 kDa. N-terminal microsequencing of an internal peptide from the 12. 6-kDa protein showed 100% homology with human macrophage migration inhibitory factor (MIF). The protein was positively identified as MIF by Western blot analysis using a specific anti-MIF antibody. Anti-MIF antibody inhibited the bioactivity found in the evaluated fraction and the conditioned medium of primary endometriotic cell cultures, and commercial recombinant human MIF displayed a high mitogenic activity for endothelial cells. Our findings reveal that MIF is released by endometriotic cells and acts as a potent mitogenic factor for human endothelial cells in vitro. This may have a considerable interest, in view of the crucial role of angiogenesis in ectopic endometrial cell growth and activity and in numerous tissues undergoing dynamic physiological changes, such as human endometrium.