The role of pseudoimmunologic reactions in the pathogenesis of byssinosis was studied. Extracts were prepared from cardroom cotton dusts and from bracts, carpels, stems, leaves, and cotton lint. Serum samples were taken from 59 cotton textile workers (SIC-2211) and 35 nonexposed volunteers. Serum samples were treated with extracts, incubated, and centrifuged. Precipitation arc patterns were determined by gel diffusion and electrophoresis. No precipitating antibodies were detected in sera of exposed workers. Serum proteins were precipitated in a nonimmunologic reaction. Treatment of serum with plant tannins did not precipitate serum proteins. Beta region proteins were identified as human beta lipoprotein, but gamma region proteins could not be identified.