SummaryIn vitrolymphocyte transformation and the precipitating antibody to house dust and mite (Dermatophagoides pteronyssinus) extract were studied in mite‐ and house‐dust sensitive patients and in non‐atopic individuals. House‐dust extract was able to induce a significant [2‐14C]thymidine incorporation into lymphocytes from house‐dust‐sensitive as well as non‐allergic individuals. Anin vitroresponse to mite extract, even if less marked, was also seen in most of mite‐sensitive patients.By the micro‐Ouchterlony and counterimmunoelectrophoresis techniques anti‐dust antibodies were found in the serum of all house‐dust‐sensitive and non‐atopic individuals tested. Anti‐mite precipitins were detected in