The efficacy of hyposensitization with standardised extract of Dermatophagoides pteronyssinus (D. pteronyssinus) conjugated to alginate and containing known amounts of antigen P1 (Conjuvac®) was tested in a double blind, placebo controlled, multi-centre study in 66 adult patients with perennial rhinitis. Patients received 11 weekly injections of increasing concentrations of Conjuvac containing from 56 × 101 to 448 × 103 IU D. pteronyssinus or placebo injections of the alginate diluent to some of which 5 μg of histamine has been randomly added This was followed by 15 monthly injections of Conjuvac or placebo. The severity of nasal blockage, sneezing and rhinorrhoea was recorded twice daily in a diary and visual analogue assessments (VAS) made at each clinic visit. Nasal provocation testing (NPT) was performed with increasing concentrations of the same extract of D. pteronyssinus as used in the hyposensitization injections, and changes in nasal airways resistance measured by passive anterior rhinomanometry. VAS was recorded and NPT was performed on entry to the study and after the fifth, ninth and final monthly injection. Conjuvac injections were well tolerated. Large local reactions (> 5 cm) occurred within 30 min in only 1% of patients but later in 23%. No systemic reactions or anaphylaxis occurred within 30 min of injections, but urticaria or worsening of asthma and rhinitis was reported later in 3% of patients. A significant improvement in nasal obstruction occurred in the Conjuvac compared to the placebo treated group (P < 0.01) and there was significant increase in the percentage of patients able to tolerate provocation with the highest concentrations of D. pteronyssinus extract after nine and 15 maintenance injections of Conjuvac compared to the placebo (P 0.02, P < 0.001). Patient use of additional therapy decreased sooner in the Conjuvac treated group but was minimal in both groups after 5 months of the study.