Cytokines are known to play a major role in mediating many of the immunological and pathological features of allergic disease. Much of our understanding of cytokine production in response to allergens has come from studying allergen-specific T cell clones following long-term in vitro culture. This has largely been due to the lack of sufficiently sensitive assays to measure allergen-induced cytokine production by freshly isolated peripheral blood mononuclear cells (PBMCs). Here we have used the polymerase chain reaction to amplify reverse transcribed interleukin-4 (IL-4) and IFN gamma mRNA expressed by allergen-stimulated PBMCs from a variety atopic individuals. Using Der p II, a major allergen of the house dust mite (HDM) Dermatophagoides pteronyssinus, we have demonstrated that cells from HDM-sensitive atopic patients (n = 12), can be induced to express either IL-4 alone (three patients), IL-4 and IFN gamma (six patients), IFN gamma alone (two patients) or neither cytokine (one patient). Cells from 13 non-atopic control individuals were also stimulated with Der p II and cytokine mRNA production was studied. None expressed IL-4, while seven of 13 transcribed IFN gamma. Our results suggest that atopic individuals have allergen-reactive T cells at various stages of differentiation, with respect to the cytokines they produce. The use of this technique will aid in the further understanding of specific cellular hypersensitivity in allergic disease.