Thirty-eight adults with allergic rhinitis have been treated with a new partially purified extract of house dust mite (Dermatophagoides pteronyssinus) in a double-blind placebo-controlled trial. Patients were randomized to active (Pharmalgen®, D. pteronyssinus) and placebo (histamine) treatment by sensitivity to D. pteronyssinus on nasal challenge. In the actively treated group nasal symptoms, assessed by visual analogue score, improved (P <0.01), sensitivity on nasal challenge with allergen was reduced (P <0.05) and weal size on skin-prick test with allergen was reduced (P <0.01), compared with the placebo group. These results occurred after 3 months of treatment. Reduction in target organ sensitivity occurred, while the serum level of D. pteronyssinus IgE rose in the active group from 14.2 to 22.5 PRU/ml (geometric mean) but did not change significantly in the placebo group. As anticipated, because of the treatment schedule used, a number of generalized allergic reactions were induced by injections, but all responded promptly and easily to treatment. These results suggest this is an effective form of therapy, which now offers us the opportunity to study the immunological mechanisms of hyposensitization and to devise a modified schedule causing fewer reactions.