The microbiological atmosphere of Danish textile mills has been studied as has the immunoglobulin content of blood from Danish mill workers. This study describes identification of antibodies reactive with a cotton dust extract in sera from Danish mill workers. Sera were obtained from 45 textile mill workers. Aqueous extracts were prepared from samples of cotton dust obtained in the USA. Antibodies were detected in the sera of 35 of the workers. The specificity of reactions was confirmed by ELISA inhibition assay. The workers were found to have made antibodies to the same component in cotton dust atmospheres as had guinea pigs exposed to cotton dust atmospheres. These findings indicate that detection of IgG antibodies in serum may be a sensitive biological monitor of cotton dust exposure.