Sera from some patients with hypersensitivity pneumonitis (HP) and lung fibrosis contain immunoglobulin G (IgG) which strongly binds to cellulose as measured by binding to paper discs. This binding is Fab-dependent. The IgG-species which binds to cellulose also binds to protein-saturated nitrocellulose membranes. This makes the detection, in patients sera, of antibodies to etiologic antigens by immunoblotting difficult. Detection by immunoblotting can, however, be performed much more precisely if patients' sera are preincubated with cellulose powder to remove cellulose-binding IgG.