An osteolytic bone tumor was found in a 12‐month‐old male NZB mouse. Macroscopically and histologically this tumor was angiosarcoma originating in bone with distant metastasis present in the spleen. The primary tumor was successfully transplanted in the back subcutaneous tissue of syngeneic mice, and the characteristics of the transplants, were found to be similar to those of the primary tumor. The ultrastructure of tumor is descri