Specific antibodies isolated by immunoabsorption on four main insolubilized allergens from Dermatophagoides pteronyssinus (DPT) had the following isotypic distribution: in 16 atopic patients, 52% IgG, 40% IgM, 8% IgA, 0.1% IgE and, in 12 non-atopic individuals, 48% IgG, 46% IgM, 6% IgA, 0.03% IgE. The ratios between geometric means of antibody values in each class (atopic vs non-atopics) were 2.4 for IgG, 2.0 for IgM, 2.8 for IgA and 66.7 for IgE. The amount of anti-DPT antibodies in IgG subclasses did not follow the usual distribution of total IgG subclasses, i.e., IgG1 greater than IgG2 greater than IgG3 greater than IgG4. In atopics the order was IgG2 greater than IgG1 greater than IgG4 greater than IgG3 and in non-atopics, IgG4 greater than IgG1 = IgG2 greater than IgG3 although 6/12 of the latter had no detectable (less than 0.5 micrograms/ml plasma) IgG4.