House Dust Extract The use of house dust extract has been limited by the weak concentration of the specific excitant in the fluid extract. Even when the extracting fluid was used in a quantity just sufficient to wet the dust, the extracts were active in the intradermal test in a dilution of at most 1:10, and they were never capable of causing a diagnostic cutaneous reaction with the scratch method. We have endeavored to concentrate the extract after dialysis against buffered saline solution by evaporation with the use of an electric fan, without heating, and by evaporation in vacuo at 37°C., with the aid of a current of nitrogen gas. Neither of these procedures accomplished a concentration of the activity of the extract. No such concentrated preparation was found capable of causing a diagnostic cutaneous reaction with the scratch technic.