There is a pressing need for in vivo models in which potential antitumor agents can be tested for their ability to inhibit the growth and metastatic spread of human sarcomas. A recent advance in this regard has been the development of a v-Ki-ras-oncogene-transformed human osteosarcoma cell line (KRIB) that efficiently colonizes the lungs of athymic nude mice when cells (1 × 10⁵) are administered by i.v. injection. In the present study, we have utilized this cell line to develop a spontaneous metastasis model in which a small number of tumor cells are injected into the tibial bones of athymic mice. When as few as 1000 KRIB cells are orthotopically implanted into the tibial bones of nude mice, bone tumors, which are radiographically and histologically similar to primary human osteosarcoma, develop within 4 weeks. Furthermore, as in the human disease, cells from these primary tumors subsequently seed the animals' lungs, resulting in reproducible and quantifiable pulmonary metastasis evident both upon gross inspection of the lungs and histologically 6 weeks after tumor inoculation. Surgical amputation of the tumor inoculation site up to 2 weeks after tumor injection prevents pulmonary metastasis, indicating that substantial local (tibial) growth and invasion of the primary tumor for at least 2 weeks is required for subsequent metastasis. Implantation of s.c. 5000 KRIB cells fails to produce local or metastatic tumors. We anticipate that this model will prove to be a powerful tool with which to study the mechanisms of human osteosarcoma growth and pulmonary metastasis, and to assess the efficacy of promising therapeutic agents.