Der fI is a major mite proteinaceous allergen found in house dust. We produced recombinant Der fI (reDer fI) in insect cells using a baculovirus expression system. Based on the molecular mass and N-terminal amino acid (aa) sequence analysis, reDer fI was found to be compromised of a mixture of two proteins, each of which includes a pro-sequence of different length. The reDer fI had IgE-binding activity at only 20% of that of native Der fI. The removal of the pro-sequence in an acidic solution drastically increased IgE-binding activity to almost the same as that of native Der fI, showing that the presence of the pro-sequence is inhibitory to the IgE-binding activity of Der fI.