Oncogenes in DNAs from human tumor cell lines have been detected by a new transformation assay. Cellular DNAs are transfected into NIH3T3 murine fibroblasts, and transformed cells are selected by maintaining cell cultures in a defined medium lacking platelet-derived or fibroblast growth factors. DNAs from eight of 17 human tumor cell lines have yielded transformants by this method. Activated cellular ras genes account for three of the transforming activities. The SAOS2 osteosarcoma cell line contains an activated oncogene distinct from 18 known oncogenes. Another cellular oncogene was detected as the consequence of a fortuitous transfection-mediated DNA rearrangement.