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A comparative immunological and microbiological study of experimental coalworkers' pneumoconiosis (CWP) was made in rats and mice subjected to long-term exposures of coal-mine dust aerosols. Such aerosols were realistically prepared at a concentration equal to the maximal level of respirable dust permitted by Federal standards and animals were exposed for lengths of time equal to human work contact. Among the factors studied were the production of IgA and lung reactive antibody, lung microflora and changes in pulmonary clearance. Additional experiments were concerned with the effects of passively administered lung antibody on the pulmonary clearance. It was found that both species responded immunologically in a similar manner to humans with CWP in that IgA levels were significantly elevated and lung reactive antibodies were stimulated. Coal-mine dust inhalation had little effect on the pulmonary inactivation of inhaled bacteria, but the concomitant occurrence of passively administered lung reactive antibody seemed to enhance the inactivation.