We have compared the effects of several amino acid treatments on the induction of ornithine decarboxylase activity and the accumulation of putrescine, spermidine, and spermine by 12-O-tetradecanoylphorbol-13-acetate (TPA) in mouse epidermis in vivo and in vitro. Incubation of isolated epidermal cells with mM concentrations of glycine, asparagine, glutamic acid, canavanine, arginine, and/or lysine inhibited dramatically the induction of ornithine decarboxylase activity by the tumor promoter. These remarkable inhibitory effects were concentration-dependent and additive. Arginine and its analog, canavanine, inhibited to the same degree TPA-induced ornithine decarboxylase activity, and potentiated to the same extent the inhibitory effects of glutamic acid, asparagine, and glycine on this enzyme. However, the inhibitory effects of arginine and canavanine were not additive. Similar alterations of tumor promoter-induced epidermal ornithine decarboxylase activity were observed in vivo when 62.5 mumol of the amino acids were injected i.p. 2 h before the topical application of 8.5 nmol of TPA to mouse skin. The results suggest the possibility that treatments with glycine, asparagine, glutamic acid, and arginine, the amino acids that were the most effective in inhibiting the tumor promoter-induced accumulation of polyamines in vivo, may reduce the tumor-promoting ability of TPA.