Recently, the immunoregulative molecule CD40 has also been introduced as a potential surface determinant of endothelial cells that can be induced by various cytokines and thus might be involved in inflammatory vascular reactions. In this study, the ubiquitous endothelial expression of CD40 within the neovascularized areas of renal cell carcinoma is demonstrated. The strong capillary expression of CD40 in 12 tumor samples is contrasted by the absence of endothelial CD40 in the corresponding tumor-free kidney specimens in which only certain tubular segments and few interstitial cells carry CD40. Northern hybridization studies confirmed the presence of CD40 RNA in cytokine-treated endothelial cells and in renal cell carcinoma, whereas no hybridization signal was obtained with normal kidney tissue. That the presence of tumor cells is pertinent to the endothelial expression of CD40 could be substantiated by in vitro experiments, when a renal carcinoma cell line and its supernatant, but not normal kidney cells, could induce CD40 on endothelial cells in culture. According to further experimental results, the carcinoma-derived, CD40-inducing factor(s) is not represented within a variety of pleiotropic cytokines including IFN-gamma, interleukin 1, interleukin 6, and tumor necrosis factor alpha, or common angiogenic factors such as basic fibroblast growth factor, vascular endothelial cell growth factor, angiogenin, and erythropoietin. The immunohistological results showing a widespread, even distribution of CD40 in tumor capillaries suggest that within renal cell carcinoma, the appearance of endothelial CD40 may also be related to angiogenesis in addition to inflammation.