A trial was carried out in 18 asthmatic patients to assess the effectiveness of a vaccine of house dust mite extract (Dermatophagoides pteronyssinus) absorbed on tyrosine. Patients were initially given 6 graduated doses of the vaccine subcutaneously at 7-day intervals. Maintenance injections were then given at 2-week intervals, the total dose being determined by clinical response. About 60% of the treated patients showed a significantly beneficial response to immunotherapy as assessed by symptomatic improvements. Seventy-two per cent of the patients required less oral anti-asthmatic therapy and 50% of the patients had a demonstrable increase in blocking antibodies. Local reactions were not severe and occurred infrequently. None of the patients developed systemic reactions to immunotherapy. Clinical response was found to depend to some extent on the dosage of antigen administered and rise in blocking antibodies of the patient's serum.