An efficient preparation of Periplaneta americana nymphae allergen, Cr PI (54 kDa) is described. It was expressed as a GST-tag fusion protein in Escherichia coli, strain BL21 (DE3). Expression of recombinant Cr PI (rCr PI), denaturation/renaturation of the inclusion bodies and the effects of protein and L-arginine concentration on inclusion body aggregation were optimized. The fusion protein was purified by affinity chromatography and size exclusion chromatography, and Cr PI fusion protein was purified to >95%. rCr PI bound strongly to IgE in the sera of individuals with cockroach allergies as shown by western blot and ELISA. Highly refolded and purified recombinant protein was obtained, providing a basis for the large-scale preparation of Cr PI allergen.