Light amplification at 6328 Å in a rhodamine-B-doped polyurethane thin film is observed. The light-guiding thin film is pumped transversely with a nitrogen laser. Superradiant light of the dye-doped film is observed to be guided by the thin film. When a He-Ne light at 6328 Å is coupled into the same thin film along the same path, amplification results. Maximum unsaturated gain on the order of 13 cm-1 was experimentally determined by two different methods in a single-mode thin-film light guide with a concentration of 3.3×10-2 mol/liter when wet.