Mammary-derived growth inhibitor (MDGI) has previously been localized in the mammary parenchyma, dependent on the stage of differentiation of the mammary gland. Here, we have elucidated the distribution of MDGI in the mammary stroma by a combined immunohisto- and cytochemical analysis with antibodies raised against MDGI. Distinct staining of capillary endothelial cells has been revealed. Although its subcellular distribution resembles former observations in secretory epithelial cells, the expression of MDGI in capillary endothelial cells clearly precedes that in secretory epithelial cells. On the other hand, no endothelial MDGI staining has been detected in bovine heart, which contains a fatty acid-binding protein almost identical to MDGI. The localization of MDGI in the mammary capillary endothelium is discussed in terms of its possible involvement in the intracellular transport of hydrophobic ligands or in the regulation of endothelial cell proliferation.