By introduction of conjugated double bonds into the long-chain fatty acid residue of the phorbol ester tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) the promoting efficacy is abolished, whereas hyperplasiogenic and irritant activities are not impaired. By means of such "disarmed" phorbol esters (with 12-O-retinoylphorbol-13-acetate, RPA, being the most suitable one) the process of skin tumor promotion can be divided into two stages. Stage 1, brought about by short-term (single) treatment with TPA, leads via an induction of cellular proliferation to an apparently irreversible change in skin which is proposed to involve the expression of the neoplastic phenotype. A subsequent long-term (multiple) treatment with RPA (stage 2), results in the appearance of papillomas. There is no positive evidence for a critical role of phorbol ester receptor occupancy and protein kinase C activation or of free radicals such as superoxide anions in stage 1. Retinoic acid inhibits stage 1 only when applied several hours prior to TPA, whereas indomethacin exhibits the strongest inhibitory effect on stage 1 when applied 3 hr after TPA. The indomethacin effect can be specifically overcome by prostaglandin F2 alpha (PGF2 alpha) and correlates with an accumulation of PGF2 alpha in skin 3-4 hr after TPA treatment. After 12-O-retinoylphorbol-13-acetate (RPA) application no prostaglandin accumulation is seen at this time. It is proposed that the expression of the neoplastic phenotype ("conversion" of initiated cells) is accomplished in the course of a PGF2 alpha-mediated metaplastic process which normally plays a physiological role in the wound response.