Clones of cells have been isolated from a culture of NIH 3T3 cells and characterized. A high degree of variation between the clones was observed in their efficiency of transformation following transfection with the ras and myc oncogenes. No correlation was found between this characteristic and either the growth rate of these cells in vitro or the efficiency of transfection, as judged by the acquisition of geneticin resistance and chloramphenicol acetyl transferase activity. Continuous maintenance of NIH 3T3 cells in culture resulted in a significant decrease in susceptibility to transformation. It is suggested that the NIH 3T3 cell line comprises a heterogeneous population of cells, and it is the balance between the various types of cells which determines the phenotype of the culture. This balance can be spontaneously disrupted while the culture is grown continuously in vitro.