Dermatophagoid.es farinae allergen Ⅰ (Der f Ⅰ ) has been extracted and purified from spent culture medium of Dermatophagoid.es farinae by affinity chromatography coupled with anti-Der f Ⅰ McAb for the first time in China. Furthermore,its physicochemical properties were analysed. By SDS-PAGE,.Der f Ⅰ was estimated to have a molecular weight of 24 kDa and appeared as only one protein band. By isoelectric focusing ,it was revealed to have pIs of 4. 8 - 7. 5 with six protein bands,suggesting that Der f Ⅰ is heterogeneous with complicated composition.