Serum proteins reacting with a pseudoimmune precipitating agent found in cardroom cotton dust and cotton stem material were identified. Specimen particles were ground and extracted separately at 4 degrees C and the supernatant fluids were sterilized and concentrated 40 fold. Antisera to cotton bracts and leaves were developed in rabbits. Human and rabbit sera were examined for immune and pseudoimmune precipitation, and sera from normal human subjects, cotton textile workers and a serum preparation lacking beta-lipoprotein were separately mixed with either the cotton dust or stem extract, incubated and centrifuged. No true precipitating antibodies to extracts of cotton bracts, carpels, stems, leaves, immature cotton lint, or cardroom cotton dust were found in the 59 human sera samples. Precipitating antibodies were produced in the rabbit immunization test. The authors note that the plant agent responsible for precipitation of the beta-lipoprotein and gamma globulin was suspected to be a polyphenic tannin.