While there are a number of experimental animal models for osteosarcoma development, the available rat systems are limited both in number and applicability. The present report concerns our attempts to establish an appropriate rat model to allow development of new experimental approaches to human osteosarcoma. A spontaneously occurring osteosarcoma (J. H. 1-OS) was found in a 35 week old male rat. The tumor could be transplanted and grew well and although the bone forming activity decreased after the 18th generation, the growth potential remained high.