Guinea pigs and rats /were exposed/ to airborne kaolin dust for 6 hours/day, 5 days/week for 1 year. Guinea pigs exposed at 23.4 mg/cu m showed slight pleural mottling. The lymph nodes contained large masses of dust cells. Rats exposed at 27.1 mg/cu m developed scattered dust foci; collagenous fibers were observed between the cells and the lymph nodes contained large masses of dust. Intratracheal injection of rats with kaolin dust was associated with similar reactions, but the coarser particles elicited a large number of foreign body giant cells. The silica content of the kaolin was not specified.