Guinea pigs of Hartley strain were sensitized with an extract of the house dust mite, Dermatophagoides pteronyssinus. Positive immediate skin reactions were obtained in more than half of the animals 6 weeks after repeated intradermal injection. Adjuvants of A1(OH)3 gel or Bordetella pertussis vaccine did not significantly enhance the production of reaginic antibody. Prausnitz-Küstner type skin test titers were even higher in guinea pigs sensitized with the mite extract without adjuvant. Circadian rhythm and difference in injection sites were not noted when assessed by skin tests on sensitized guinea pigs. Passive hemagglutination test detected serum antibodies and some animals showed respiratory distress on the challenge with the aerosolized extract. This system would be an another useful laboratory model of acute respiratory allergy of the most important inhalant allergen, the house dust mite.