A cell line (ISO-HAS) has been established from tumor tissue of a human hemangiosarcoma arising on the scalp by the use of conditioned medium from a murine-phenotypic angiosarcoma cell line (ISOS-1). Cells have been cultured for more than 2 years with up to 100 passages. The cells retained endothelial-cell properties, such as a characteristic cobblestone appearance at confluency, contact-inhibited growth, active uptake of acetylated low-density lipoprotein labeled with 1,1-dioctadecyl 1,3,3,3,3-tetramethyl-indocarbocyanine perchlorate (DiI-Ac-LDL) and CD31 expression. However, they were weakly positive for von-Willebrand-factor (vWf) antigen and for binding of Ulex europaeus agglutinin-I (UEA-I) lectin, and lacked tube-formation activity. These findings indicate that ISO-HAS is a poorly differentiated endothelial cell line. ISO-HAS cells showed accumulation of p53 protein in the nuclei, and a new-typed p53-gene point mutation was found in exon 7 at codon 240. When inoculated s.c. into severe-combined-immunodeficiency (SCID) mice, the cells showed solid-tumor growth that caused death. These properties suggest that ISO-HAS is a malignant endothelial cell line with high tumorigenicity.