Recently, we cloned a new Dermatophagoides pteronyssinus allergen homologous with glutathione S-transferase. IgE radioimmunoassays against the Escherichia coli lysate of the recombinant clone reveal that 40% of mite allergic subjects recognize the mite glutathione S-transferase allergen. Of these sera, greater that 50% have reactivity with the recombinant mite glutathione S-transferase that is greater than 10 times the result observed with a normal control. Immunoblotting studies with sera from patients that recognize the recombinant protein reveal IgE binding to a 26-kDa protein on immunoblots of reduced mite protein extracts. The 26-kDa IgE-binding band observed on immunoblots of reduced mite proteins, corresponding to the cloned protein, is a separate and unique allergen from the 25-kDa Der p I as the apparent electrophoretic molecular mass of Der p I shifts from 25 to 30 kDa under conditions of reduction.