Cotransfection of rat embryo fibroblasts with c-myc and activated H-ras oncogenes is one experimental model of the multistep oncogenesis associated with p53 mutations and aneuploidy. Using the model, we found that selection processes, e.g., r- and K-selection, affect emergence of p53 mutants and tetraploids. Culture optimum for logarithmic growth (r-selection) selected p53 mutants as they proliferated rapidly, while in confluent culture (K-selection) tetraploids emerged regardless of the p53 status. Transfection of the mutated p53 gene with dominant negative functions eradicated untransfected cells under both r- and K-selection. However, these p53 mutants can be eradicated under K-selection by cells with normal p53 function and that had been selected under prolonged K-selection. The presence of competitors and the type of selection should determine whether or not p53 mutants and/or tetraploids predominate. These observations strengthen the importance of selection processes in case of cancer.