A survey of the work with Ig response to allergens carried out previously reveals an allergen-specific response both by IgE and all of IgG subclasses. Response of non-sensitive people is characterized by the appearance of a variety of the IgG subclasses. We have reexamined ragweed and Amb a 1 specific Ig response in 54 nonsensitive and 147 atopic or atopic- allergic people using a new inverse sandwich immunoassay allowing discrimination based on antibody affinity. We show that nonsensitive people present no, 0 out of 54, Ig response with affinities higher than K(a) 107 M-1. The subpopulation of 66 atopics who never have experienced desensitization responds vigorously and solely (56 out of 66) with genes of the sequence γ2-α2. Only ten showed an additional weak response from γ1- α1. This suggests a possible association between the atopic state and selective activation of part of the gene sequence.