A human type I IL-1 receptor expression plasmid has been constructed and used to transfect human melanoma cells (A375-5), which have been shown to be unresponsive to the antiproliferative effect of IL-1. Five stable transfectant cell lines have been established, of which three are sensitive and the other two resistant to the anti-proliferative effect of IL-1. All the transfectant cell lines, but not progenitor A375-5 cells, expressed functional type I IL-1 receptors and could produce IL-6 in response to IL-1, suggesting that the unresponsiveness of A375-5 melanoma cells is exactly due to an IL-1 receptor deficiency. The three IL-1-sensitive stable transfectant cell lines expressed much more type I IL-1 receptor than the IL-1-sensitive A375-6 cells, thus they are expected to be useful for investigating the signal transduction pathway of IL-1-induced growth inhibition in melanoma cells. The two resistant cell lines produced comparable amounts of IL-6 in response to IL-1, as sensitive cell lines did, indicating that IL-6 induction does not play a major role in IL-1-induced growth inhibition in these melanoma cells. The possibility of an IL-6 receptor and/or IL-6 receptor signaling deficiency was ruled out as the IL-1-sensitive and -resistant transfectants responded similarly to a high dose of exogenous human recombinant IL-6. Examination of the ornithine decarboxylase (ODC) activity of recombinant human IL-1 alpha treated cells showed that all the sensitive but none of the resistant cell lines could down-regulate their ODC activity. These results suggest that IL-1-induced ODC activity down-regulation is an important step in the signal transduction pathway of IL-1-induced growth inhibition of melanoma cells.