The JB6 mouse epidermal cell system has been used extensively as an in vitro transformation model for the study of tumor promotion. The standard JB6 cell assay for promotion of transformation is carried out in soft agar or other anchorage independent conditions. The present study was directed to the development of an in vivo model to distinguish the promotion resistant (P-) and promotion sensitive (P+) progression phenotypes. Results indicate that the grafting assay distinguishes P- and P+ cells in vivo with P+ but not P- cells forming tumors within 7-9 weeks. Expression of dominant negative mutant jun TAM67 blocks both anchorage independent transformation response and graft bed tumor formation by P+ cells, suggesting that the requirement for AP-1 activation in transformation now applies in vivo. Expression of mutated p53 produced a gain of P+ phenotype in P- cells in vitro, but not in vivo. Histochemical and Northern blot analysis for expression of various keratinocyte markers revealed no evidence for expression, suggesting a loss of keratinocyte markers following establishment in culture. In summary, the skin-grafting assay described in this study appears to be a valid in vivo assay for distinguishing the preneoplastic progression phenotypes represented by JB6 P- and P+ cells.