Skin carcinogenesis can be operationally and mechanistically divided into at least three major stages - initiation, promotion and progression. Variations among stocks and strains of mice to susceptibility to multistage skin carcinogenesis appear to be more related to alterations in tumor promotion than tumor initiation; however, the critical events have not been determined. In the mouse skin model the first stage is thought to involve the interaction of a tumor initiator with the genetic material of stem cells leading to an irreversible alteration in some aspect of growth control and/or differentiation, probably activating the Ha-ras oncogene. Some skin tumor promoters such as the phorbol esters, indole alkaloids, and polyacetates, appear to act through protein kinase C leading to specific phosphorylation of cellular proteins whereas others such as okadaic acid class of compounds appear to act through phosphatases also leading to an increase in phosphorylation. In addition, other types of tumor promoters such as peroxides, benzo(e)pyrene, and chrysarobin may act through a free radical mechanism. Regardless of the type, the major effect of the skin tumor promoters appears to be the specific expansion of the initiated stem cells in the skin. There is a very good correlation between the abilities of tumor promoters to induce a sustained hyperplasia and their tumor promoting activities. This appears to occur by both direct and indirect mechanisms involving the loss of glucocorticoid receptors, differentiation alterations, a direct growth stimulation of the initiated cells and/or selective cytotoxicity. A number of growth factors have recently been found to be increased during tumor promotion and may be responsible for the increase in cell proliferation. An inhibition of cell-cell communication and stimulation of differentiation of non-initiated cells appear to be important indirect mechanisms of further expanding the initiated cell population. The appearance of GGT and keratin 13 (K13) and the lack of expression of K1 and K10 were found to be good markers for skin tumor progression. These alterations occur at the time papillomas change from a diploid to aneuploid state which is mainly due to trisomies of chromosome 6 and 7. In order to evaluate a casual role for GGT in skin tumor progression, a functional GGT cDNA was transfected into two of our cell lines which normally produce papillomas when grafted into the skin of nude mice. The GGT positive cells and the vector transfected cells (controls) from one of the cell lines were cloned and injected into nude mice and placed into transplantation chambers.(ABSTRACT TRUNCATED AT 400 WORDS)