The specificity of newly generated IgE antibodies (Abs) to the house dust mite, Dermatophagoides pteronyssinus, in longitudinal serum samples from 18 young children with an increased risk for IgE-mediated allergy was studied. The first IgE Ab response to house dust mite was detected early in life (mean age, 32 months; range, 11 to 60 months). For 83% of the children, more than half of the newly generated IgE Ab response to house dust mite was directed against components distinct from the major allergens, Der p I (Pl) and Der p II (DpX). These results suggest that the early IgE Ab response to house dust mite is induced by components distinct from the major allergens, Der p I and Der p II.