An experimental model for osteosarcoma was developed in which, after Moloney murine sarcoma virus was injected into the tibial marrow space of three strains of inbred neonatal rats, a highly malignant neoplasm arose within ten days. This tumor was readily maintained in tissue culture and was transplantable to adult rats. It arose in the metaphyseal marrow of several bones, was locally invasive, metastasized to the lungs, and histologically resembled osteosarcoma.