A comparison was made of immune responses to house dust mite allergens in symptomatic patients from paediatric and adult allergy clinics. IgE-specific immune reactivity to Dermatophagoides pteronyssinus was examined by Western blotting and, for Der p I and Der p II. by radioimmune dot-blot using purified allergens. Nineteen mite-sensitive children (mean age 9 years) and 26 adults (mean age 31 years) were compared. Positive IgE responses by dot-blot were found to Der p I and Der p II in 79% of children, whereas reactivity was only present in 23% and 19% respectively of adults, and densitometry indicated a weaker response. In children. Western blotting indicated that the majority of the serologic reactivity was directed to Der p II (17/19, 89%) or a 100 kDa fraction whereas in adults, reactivities were generally directed to other fractions with only 15/26 (58%) recognizing Der p II. Consistent with results of others, a fraction corresponding to Der p I was poorly detected on Western analysis despite positive dot-blots. To determine whether the relative lack of IgE serological immune reactivity in adults was associated with a similar lack of cellular recognition, T-cell proliferation studies were performed, also using purified allergens. Interestingly, these revealed that cellular responsiveness, without serological reactivity, was present in 29% of subjects to Der p I and 50% to Der p II. Proliferative responses were evident in all individuals with specific IgE.