Inhalation of cotton dust has been found to result in specific antibody production in guinea pigs. The possible use of the antibody response as a biologic marker of exposure was explored by exposing animals on a number of occasions to atmospheres of cotton dust. Exposure on one occasion to 30 mg/m³ did not result in antibody production whereas exposure on 5 or more occasions did. Antibody titer, measured by ELISA, was dependent on the cotton dust concentration, not the number of exposures. Extrapolation of the concentration-response data indicated the sensitivity of the response would be 10 ug/m³ dust exposure. This immunologic IgG response appears to be a sensitive, longlasting marker of cotton dust exposure.