AbstractMouse epidermal cell lines have been identified which respond to tumor‐promoting (but not nonpromoting) phorbol esters with an irreversible shift in anchorage independence, an in vitro marker of neoplastic phenotype. This response may be analogous to a later stage of tumor promotion in vivo. The shift occurs at TPA concentrations as low as 0.1 ng/ml (1.6 × 10−10M). The specificity of the soft agar growth response is not limited to phorbol esters but extends to nonphorbol plant diterpenes such as mezerein, to detergents, to polycyclic hydrocarbons present in cigarette smoke, and to some growth factors. All of the above classes of compounds have been previously shown to have tumor‐promoting and/or cocarcinogenic activity in mouse skin in vivo. Clonal heterogeneity for TPA responsiveness has been found. Clones which were highly responsive to phorbol esters were also highly responsive to other classes of promoters, indicating their usefulness both for promoter detection and mechanism studies. The anchorage‐independence response to TPA was inhibited by a series of retinoids whose activity paralleled that for inhibiting tumor promotion in vivo. Both retinoid inhibition and clonal heterogeneity for promoter response are being utilized to study determinants of preneoplastic pro