We have studied the precipitation test as a diagnostic procedure for hypersensitivity pneumonitis or associated allergic states using Sitophilus granarius (SG) antigen. The incidence of positive reaction in the precipitation test against SG is extremely high in children and adults, not only asthmatics, but also in normal, healthy subjects. In experiments with radioimmunoassay using SG paper discs and 125I-labelled IgG fragments, SG was more strongly bound to the Fab than to the Fc fragment. However, the binding was not mediated by a true antigen-antibody reaction because the reaction was inhibited by both kappa- and lambda-type Bence-Jones proteins. Nonspecific immunoglobulin binding substance (IBS) was separated from crude SG extract by affinity chromatography using IgG immunosorbent. IBS was monocomponent on polyacrylamide gel electrophoresis in sodium dodecyl sulfate, and the molecular weight was about 12,000-14,000. We believe that IBS might be a valuable tool for immunological experiments, comparable with staphylococcal protein A.