Twenty-five atopic children under 11 years of age were studied, using skin and RAST tests, for their specific IgE response to four species of pyroglyphid house dust mites, Dermatophagoides pteronyssinus, D. farinae, D. microceras and Euroglyphus maynei. All of the children were sensitive to D. pteronyssinus, 20 (80%) of these children were also sensitive to D. farinae and D. microceras, and 16 of the latter (64%) were also sensitive to E. maynei. Dust samples from various sites in the homes of the children revealed D. pteronyssinus in all homes studied but no D. farinae or D. microceras. E. maynei, although identified, was not present in significant numbers in any site. A control group of 20 atopic children of similar age who were not sensitive to house dust mite allergens had a similar exposure to the four mite species. These results suggest that factors in addition to mite exposure are important in the development of specific IgE responses to house dust mites.