Sixteen healthy horses with no history of skin or respiratory disease were used for an intradermal testing (IDT) threshold study, in order to determine the concentrations of 13 commercial allergenic insect extracts most appropriate for IDT. Five dilutions of each extract were used, which included the manufacturer's recommended concentrations for equine IDT, plus one dilution higher and three lower than these standard concentrations. Allergens tested included caddisfly (Trichoptera spp.), mayfly (Ephemeroptera spp.), horsefly (Tabanus spp.), deerfly (Chrysops spp.), fire ant (Solenopsis invicta), black ant (Camponotus pennsylvanicus), cockroach mix (Periplaneta americana and Blattella germanica), mosquito (Aedes aegypti), house fly (Musca domestica), moth (Heterocera spp.), flea (Ctenocephalides canis/C. felis), Culicoides variipennis and Culicoides nubeculosis. Two separate methods were used to calculate the allergen concentration for each insect extract where the normal horses, as a group, ceased to show false-positive ('irritant') reactions. 'Irritant' threshold concentrations were determined for 9/13 of these allergens, whereas the other 4 were undetermined due to either insufficient reactivity (flea, C. variipennis) or excessive reactivity (black ant, moth) to the concentrations tested. Recommended concentrations for future use in equine patients with suspected insect hypersensitivity include: 125 pnu mL(-1) (mayfly); 250 pnu mL(-1) (caddisfly, horsefly, deerfly, fire ant, house fly); 500 pnu mL(-1) (cockroach); 1000 pnu mL(-1) (mosquito); and 1:10 000 w/v (C. nubeculosis).