Nine of the 15 neonatal CBA mice injected intramuscularly with a Moloney concentrate containing FBJ virus developed tumours: likewise 5 of 7 CBA neonates injected intraperitoneally with a cell-free filtrate derived from a transplanted tumour of the former group.Of soft tissue origin, these FBJ sarcomata have a characteristic histological appearance and are of low grade malignancy. Although occasional islets of cartilage osteoid and bone were noted, these were regarded as indicative of evolutionary metaplasia in the collagenous matrix of pleomorphic fibroblastic sarcoma. No tumour was acceptable as osteosarcoma of conventional type and osseous origin. There were, however, additionally 2 minute spindle cell sarcomata arising in femoral periosteum and non-neoplastic periosteal proliferation was observed. The differences of these FBJ fibroblastic sarcomata from murine osteosarcoma-either spontaneous or induced by Sr 90-are emphasized. Furthermore, their deviation from the structural pattern and behaviour of human osteosarcoma is discussed.