A new, to our knowledge, method of modifying methylene bluedichromated-gelatin (MBDCG) holograms by use of ninhydrin andremoval of their amino groups and carboxyl groups, which stronglyadsorb humidity, has been developed. The modified MBDCG hologramsare stable for 1000-line/mm gratings (diffractional efficiency of95%) in a high-humidity environment (relative humidity of80%-85%). The modifying reaction does not affect thediffraction efficiency of the MBDCG holograms. It is demonstratedthat the amino groups and carboxyl groups of the side chains in thegelatin do not undergo the chelation reaction with Cr iii in the sensitive area.