Human endothelial cells derived from the umbilical vein were transformed with SV40 virions. A cell line subcultured for over 60 serial passages was characterized in comparison with its untransformed counterpart which was culturable for less than five passages. The SV40-transformed human endothelial cells, designated SV-HUVEC, were positive not only for tumor (T) antigen specific to the SV40-transformed cell, but also for two markers of endothelial cells, Factor VIII-related antigen and a receptor for Ulex europaeus agglutinin I. By transformation the growth potential of the human endothelial cells was increased and their serum requirement was decreased. The SV40-transformed endothelial cells were, however, unable to form colonies in soft agar or to form tumors in athymic nude mice, although a small nodule was produced at the site of inoculation. Subcultivation of these cells up to the 62nd passage eventually resulted in crisis and loss of further cell division. Thus, the human endothelial cells were transformed by SV40 while retaining certain normal functions but without showing tumorigenicity.