Sphingosine-1-phosphate receptor 1 (S1P(1)), a receptor for sphingosine-1-phosphate, has been shown to play an important role in the migration, proliferation, and survival of several types of cell including endothelial cells. Given that S1P(1) signaling could serve as a therapeutic target, we evaluate the expression of S1P(1) in formalin-fixed and paraffin-embedded sections from human tissues, using automated immunostainers (Ventana). The specificity of the polyclonal rabbit anti-human S1P(1) antibody used in this study was defined by immunostaining of the vasculature in S1P ( 1 ) ( -/- ) and S1P ( 1 ) ( +/- ) mouse embryos. The antibody stained the newly formed vasculatures ex vivo in a serum-free matrix culture model using rat aortic rings. In human specimens, S1P(1) was strongly expressed on the cell surface membrane of endothelial cells of blood and lymphatic vessels in all tissues examined. The expression of S1P(1) was confirmed by the flow cytometric analysis and real time RT-PCR of an angiosarcoma cell line. This study indicates that S1P(1) can be used as an immunohistochemical marker for human tissue endothelial cells.