Bone cancer can be induced by radionuclides that localize in the skeleton. Histologically, these experimentally induced tumors resemble those found naturally in man; they range from densely ossified osteogenic sarcomas to osteolytic tumors with giant cells and only a small osteoid component. Fibrosarcomas and hemangiosarcomas also can occur in some species. It has not been possible to determine the dose in terms of absorbed energy necessary for bone-tumor induction because radionuclides are not deposited uniformly, and they diminish in amount with time. Also, the precise time when irreversible noeplastic change occurs is not known. With X-rays, however, 500 rads delivered to the endosteal surface of a mouse femur has been shown to cause osteogenic sarcoma. Bone tumors can be induced in mice by viruses. FBJ osteosarcoma virus and RFB osteoma virus were obtained from spontaneous tumors; FBR osteosarcoma virus came from a radiation-induced tumor. All three are RNA viruses with C-type particle morphology, and they are propagated by injecting cell-free extracts of virus-induced tumor. All three are RNA viruses with C-type particle morphology, and they are propagated by injecting cell-free extracts of virus-induced tumor into newborn mice. Interaction studies with bone-seeking radionuclides and these viruses have led to the hypothesis that radiation produces cancer by inactivating a viral inhibitor. There is also evidence of a bone tumor virus in the human disease. The injection of cell-free extracts of human bone cancer into newborn Syrian hamsters has induced a variety of mesenchymal tumors at a rate significantly higher than in the control hamsters. Sixty tumors of this type, including 20 osteosarcomas, 11 fibrosarcomas, and 9 osteomas, have been diagnosed so far in experimental animals; in control hamsters there has been only one, a fibrosarcoma. Immunofluorescence assays and cytotoxicity studies indicated that these hamster tumors carried a human antigen.