The purpose of this project was to determine the effects of fly-ash inhalation on the ability of animals to resist infections, neoplastic growth, and the development of hypersensitive responses. Mice were exposed to fly ash from two different sources, carbon black, and filtered ambient air only. Following exposures of various lengths (days to months), a variety of tests were done to determine the immunologic competence of lymphocytes, neutrophiles, and macrophages. Fly ash had a suppressive effect on the ability of mice to respond to Escherichia coli antigens given by aerosol; this suppression was much less severe than that following exposures to carbon black. Fly ash had little effect on the ability of B and T lymphocytes to respond to mitogens and to be stimulated for cytolytic response against tumor cells. The cellular response of BCG-sensitized mice to purified protein derivative of tuberculin was enhanced. The effect of fly ash on macrophages was more pronounced, involving decreased phagocytic activity, decreased antibody-dependent cytolosis, increased cytoxic activity against tumor cells, and decreased ability to activate T cell mitogenesis.