A transplantable ascites-forming osteosarcoma (J. H. 1-AOS) derived from the 35th generation of spontaneous osteosarcoma, J. H. 1-OS, grown in Fischer 344 syngeneic rats was established. Tumorigenicity, histochemical and ultrastructural characteristics were investigated. Rats carrying the ascites form osteosarcoma died of cachexia about 15 days after transplantation, 1.5-2.5 x 10(6) cells/ml of tumor cells generally being involved in the ascites and tumor nodules formed in the mesentery. After inoculation into the back subcutaneous space, tumor growth was very rapid. Small round cells were detected in the Giemsa stain smear, and although osteoid formation was histologically lacking, cell surface alkaline phosphatase activity was noted both light and electron microscopically. Polyacrylamide gel electrophoresis demonstrated that alkaline phosphatase (Al-p) extracted from this tumor was consistent with Al-p from rat fetal calvaria. Thus maintenance of osteogenic potential is suggested for these ascites osteosarcoma cells, indicating their usefulness for further studies of biological behaviors of this tumor type.