High-molecular-weight DNAs from 5 bladder carcinomas were used in transfection of mouse NIH3T3 cells. The manifestation of heterologous oncogene(s) expression in NIH3T3 cells was morphological transformation very often accompanied by changes in growth characteristics of recipient cells. In DNA samples from secondary NIH3T3 transformants human c-Ha-ras and c-sis sequences were identified. In some secondary transformants these sequences were expressed. On the basis of change of the growth characteristics of some secondary transformants we could expect the integration and expression of another human gene(s) for growth factor or growth factor receptor or even activation of mouse genes. We did not manage to identify any Alu sequences in some secondary transformants carrying human c-Ha-ras sequences. On the other hand, it has not been revealed yet that BamHI DNA fragments carrying c-Ha-ras gene contained any Alu sequence. So, the identification of Alu sequences does not have to be the first step in investigation of DNA samples from NIH3T3 transformants.