The mite Blomia tropicalis is a potent source of allergens in tropical and subtropical regions. So far, most of these allergens have only been studied by immunoblotting. To characterize them at the molecular level, a lambda gt11 complementary DNA library was constructed from messenger RNA isolated from whole B. tropicalis mites. This library was screened by using pooled sera from patients allergic to B. tropicalis in a plaque IgE radioimmunoassay. A B. tropicalis IgE-positive clone (Bt-M) was selected for immunologic studies. After subcloning into pBluescript (Stratagene, La Jolla, Calif.), it produced a sequence of 310 bp, with a probable amino acid sequence of 72 residues for the expressed peptide. The recombinant protein was transferred to nitrocellulose filters and probed with 100 sera from patients allergic to B. tropicalis. Forty-seven percent of sera reacted with the recombinant allergen. Immunoblottings performed with allergic serum and B. tropicalis-affinity-purified IgE demonstrated that the recombinant protein shares allergenic epitopes with the 11-13, 14, and 16 kd native allergens of B. tropicalis, which are known to be important allergens of this mite.