The allergenic similarity of the pyroglyphid mite D. pteronyssinus and the glycyphagid mite L. destructor was investigated with a new immunoblotting inhibition technique allowing simultaneous comparison of several allergens. Extracts of D. pteronyssinus and L. destructor were separated by SDS-PAGE and electroblotted to nitrocellulose (NC). A serum pool containing IgE specific to the major allergens in both mites was mixed with serially diluted extracts of D. pteronyssinus and L. destructor and incubated with the mite allergens of NC. The inhibition of the IgE binding to NC was evaluated by densitometric scanning and percentage inhibition was calculated. The IgE antibodies to the 25-kD component in D. pteronyssinus, were inhibited to the same degree by extracts of D. pteronyssinus and L. destructor. Another major allergen component in D. pteronyssinus (16 kD) was also inhibited by L. destructor extract but to a lesser degree: 400 times more of the heterologous than of the homologous extract was needed for 50% inhibition. To produce 50% of heterologous inhibition of the two major allergen components at 15 and 53 kD of L. destructor, 2000 and 10000 times more, respectively, of D. pteronyssinus than of L. destructor extract were needed. Two minor allergen components of L. destructor showed some cross-reactivity with D. pteronyssinus. However, L. destructor was a stronger inhibitor of D. pteronyssinus than vice versa, probably because the sera were obtained from persons more sensitized to L. destructor than to D. pteronyssinus.