Under contract to the Smelter Environmental Research Association, this laboratory has examined rat bone marrow and peripheral blood nuclei in metaphase for chromosomal aberrations after exposure to either one of several inorganic or organic arsenic compounds, cadmium oxide, cadmium sulphate, lead oxide or a composite flue dust. These compounds are implicated as mutagens and possibly carcinogens. Metaphase cytogenetic studies assay for structural damage to the DNA strand within chromosomes as an indicator of mutagenic events and possible genetic health hazard to the exposed population. This interim report presents the results of the bone marrow assay.