DNA transfection analyses using the tumorigenicity assay were performed on seven human acute myelocytic leukemia (AML) cell lines. DNAs from all cell lines induced tumors in nude mice. Respective transforming sequences could be identified as activated N-ras genes in AML cell lines THP-1, KG-1 and Rc2a. We observed the transfer of five putative novel transforming sequences from DNAs of cell lines U937, ML-1, HEL and CTV-2. These sequences appear to be unrelated to each other and previously known oncogenes. Transfections containing these transforming genes show distinctive biological and morphological characteristics. Our data suggest that the transforming sequences reported here became activated during the transfection process rather than the AML cell line.