Carbodiimides were employed for cross-linking of gelatin through amide bond formation to avoid any foreign bond incorporation into the cross-linked gelatin molecules. Cross-linking of gelatin was performed not in aqueous solution but in the form of a film under a heterogenous condition. Ethanol-water mixtures were used as the reaction medium to prevent dissolution of gelatin films. The optimal ethanol concentration in the ethanol-water mixtures was around 80 vol%, and the water content of the cross-linked gelatin film attained after swelling with water at 25 degrees C was as low as 55 wt%. A water-soluble carbodiimide (l-ethyl-3[3-dimethylaminoprophyl] carbodiimide) was more effective for gelatin cross-linking than a water-insoluble carbodiimide (1,3-dicyclohexyl carbodiimide). The optimal temperature for cross-linking with the water-soluble carbodiimide ranged between 15 and 25 degrees C. It was concluded that gelatin cross-linking with carbodiimides is as effective as that with glutaraldehyde, which is most widely used despite the toxicity, at least, so far as the water content of cross-linked gelatin film is concerned.