Peripheral blood mononuclear cells (PBMC) from the majority of the allergic patients that we tested who were skin test-sensitive to the house dust mite Dermatophagoides pteronyssinus (D. pt.) (46/67) showed significant proliferation in response to the purified major allergen Antigen P1; PBMC from 14/15 nonsensitive controls showed no significant response. Optimal responses were seen with 10 micrograms Antigen P1/ml, but 21 of 43 patients tested showed significant proliferation at 0.01 microgram Antigen P1/ml. The results of three types of experiments showed that the responding cells were T cells, predominantly of the helper phenotype. First, purified T cells in the presence of irradiated or mitomycin C-treated antigen-presenting cells showed good proliferation to Antigen P1. Secondly, flow cytometry analysis showed a progressive increase in the percentage of viable cells bearing the Leu-3a marker--up to 88% by day 7--and demonstrated that the larger, blast-transformed cells bore this marker. Finally, interleukin 2 production was demonstrated in antigen-stimulated cultures at days 3 to 5, i.e., about 2 days before the peak of proliferation. When patients were grouped according to their disease symptoms, no clear differences were seen between the T cell responses of patients with asthma, eczema, or rhinitis, or with asthma and eczema, although patients with rhinitis alone tended to show weaker responses. Overall, there was a significant correlation between serum IgE antibody to Antigen P1 and T cell proliferation (rs = 0.579, p less than 0.001); however, excluding individuals with no IgE antibody, the quantitative correlation was poor (rs = 0.26, p less than 0.1). These results indicate that most patients showing immediate hypersensitivity to D. pt. have circulating T cells sensitized to Antigen P1. These sensitized T cells probably act as helper cells for antibody production, but may also play a role in the pathogenesis of allergic lesions and in the delayed or chronic symptoms of these allergic diseases.