Early (twice) and late (eighth) passage nonestablished rat embryo fibroblasts exhibit dramatically different responses to microinjection of the oncogenic form (T24) of the ras protein. Late passage cells respond like established cultures in that they both synthesize DNA and divide in response to the oncogenic protein. However, early passage cells are shown to be much less responsive to the introduction of this protein in terms of the threshold concentration required to elicit a mitogenic response. Furthermore, conditioned medium from late passage cells can both potentiate the effect of ras in early passage cells and stimulate colony formation in soft agar. In addition, we show that human cells can respond to microinjection of the ras oncogene protein by synthesizing DNA.