Abstract. Five clonal cell lines were established from a spontaneous BALB/c mouse osteosarcoma, and characterized. Four of these lines showed some similarities in morphology, in vitro growth properties, production of collagenous and noncollagenous extracellular matrix proteins and osteogenic differentiation. The cells formed colonies with characteristic differences in size and morphology in soft agar, and osteogenic sarcomas and metastases in syngeneic mice after transplantation. Ultrastructurally, cells in the transplant tumours showed marked osteogenic features. There were no osteoclast-like cells. The fifth cell line had somewhat different characteristics. All five lines expressed infectious endogenous murine leukemia viruses. Increased c-myc protoon-cogene expression was found in one cell line and c-fos expression at different levels in all lines. There was only very low expression of c-Ha-ras and no expression of c-Ki-ras and c-sis. DNA analysis showed the presence of newly acquired proviral genomes integrated at different sites in the cellular DNA. The results show that distinct osteogenic neoplastic subclones can be obtained from a primary mouse osteosarcoma. Although the clones exhibited an appreciable morphological, functional, and molecular diversity they retained the basic pathogenic properties of the tumour from which they were derived.